The date of this prospectus supplement is August 13, 2013

TABLE OF CONTENTS

i

Table of Contents

ABOUT THE PROSPECTUS

        This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of common stock and also adds to and updates information contained in the accompanying prospectus. The second part is the accompanying prospectus, which provides more information about the common stock we may offer from time to time. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control.

        You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to buy, any shares of our common stock by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of our common stock. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

Forward-Looking Statements

        Information contained in this prospectus supplement and the accompanying prospectus may contain forward-looking statements, which can be identified by the use of forward-looking terminology such as "may," "will," "expect," "intend," "anticipate," "estimate," or "continue" or the negative thereof or other variations thereon or comparable terminology. The matters described in the sections titled "Supplementary Risk Factors" in this prospectus supplement and "Risk Factors" in the accompanying prospectus and certain other factors noted throughout this prospectus supplement and the accompanying prospectus constitute cautionary statements identifying important factors with respect to any such forward-looking statements, including certain risks and uncertainties that could cause actual results to differ materially from those in such forward-looking statements. We undertake no obligation to revise or update any forward-looking statements but advise you to consult any additional disclosures that we may make directly to you or through reports that we may file in the future with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K. We note that the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995 does not apply to statements made in this prospectus supplement or the accompanying prospectus.

ii

Table of Contents

S-1

Table of Contents

S-2

Table of Contents

S-3

Table of Contents

S-4

Table of Contents

S-5

Table of Contents

S-6

Table of Contents

S-7

Table of Contents

S-8

Table of Contents

S-9

Table of Contents

S-10

Table of Contents

S-11

Table of Contents

SUPPLEMENTARY RISK FACTORS

        Investing in our securities involves a number of significant risks. In addition to the other information contained in this prospectus supplement and the accompanying prospectus, you should consider carefully the following supplementary risk factors together with the risk factors beginning on page 15 of the accompanying prospectus before making an investment in our securities. The risks set out below and in the accompanying prospectus are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the events described herein or in the accompanying prospectus occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline, and you may lose all or part of your investment.

         It is unclear how increased regulatory oversight and changes in the method for determining LIBOR may affect the value of the financial obligations to be held or issued by us that are linked to LIBOR, or how such changes could affect our results of operations or financial condition.

        As a result of concerns about the accuracy of the calculation of LIBOR, a number of British Bankers' Association, or BBA, member banks entered into settlements with certain regulators and law enforcement agencies with respect to the alleged manipulation of LIBOR, and there are ongoing investigations by regulators and governmental authorities in various jurisdictions. Following a review of LIBOR conducted at the request of the U.K. government, on September 28, 2012, recommendations for reforming the setting and governing of LIBOR were released, which are referred to as the Wheatley Review. The Wheatley Review made a number of recommendations for changes with respect to LIBOR, including the introduction of S-5 statutory regulation of LIBOR, the transfer of responsibility for LIBOR from the BBA to an independent administrator, changes to the method of the compilation of lending rates and new regulatory oversight and enforcement mechanisms for rate-setting and a reduction in the number of currencies and tenors for which LIBOR is published. Based on the Wheatley Review and on a subsequent public and governmental consultation process, on March 25, 2013, the U.K. Financial Services Authority published final rules for the U.K. Financial Conduct Authority's regulation and supervision of LIBOR, which are referred to as the FCA Rules. In particular, the FCA Rules include requirements that (1) an independent LIBOR administrator monitor and survey LIBOR submissions to identify breaches of practice standards and/or potentially manipulative behavior, and (2) firms submitting data to LIBOR establish and maintain a clear conflicts of interest policy and appropriate systems and controls. The FCA Rules took effect on April 2, 2013, and on July 9, 2013, NYSE Euronext was chosen to serve as the independent LIBOR administrator commencing in 2014. It is uncertain what additional regulatory changes or what changes, if any, in the method of determining LIBOR may be required or made by the U.K. government or other governmental or regulatory authorities. Accordingly, uncertainty as to the nature of such changes may adversely affect the market for or value of any LIBOR-linked securities, loans, derivatives and other financial obligations or extensions of credit held by or due to us or on our overall financial condition or results of operations. In addition, any further changes or reforms to the determination or supervision of LIBOR may result in a sudden or prolonged increase or decrease in reported LIBOR, which could have an adverse impact on the market for or value of any LIBOR-linked securities, loans, derivatives and other financial obligations or extensions of credit held by or due to us or on our overall financial condition or results of operations.

S-12

Table of Contents

USE OF PROCEEDS

        The net proceeds from the sale of the 4,000,000 shares of common stock in this offering are $114,336,000, and $131,516,400 if the underwriter's option to purchase additional shares is exercised in full, after deducting the underwriting discount and estimated offering expenses payable by us.

        We intend to initially use the net proceeds from this offering to repay outstanding debt borrowed under our $372.5 million Credit Facility. However, through re-borrowing of the initial repayments under our $372.5 million Credit Facility, we intend to use the net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, to make investments in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, to pay our operating expenses and other cash obligations, and for general corporate purposes. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in interest bearing deposits or other short-term instruments. See "Risk Factors—Risks Relating to Our Securities—We may be unable to invest a significant portion of the net proceeds from an offering or from exiting an investment or other capital on acceptable terms, which could harm our financial condition and operating results" in the accompanying prospectus.

        At August 9, 2013, we had approximately $246.0 million outstanding under our $372.5 million Credit Facility. Our Credit Facility matures in September 2017, unless extended, and bears interest, at our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Amounts repaid under our $372.5 million Credit Facility will remain available for future borrowings. As of June 30, 2013, the interest rate on our $372.5 million Credit Facility was 2.69%.

        Affiliates of Raymond James & Associates, Inc., RBC Capital Markets, LLC and BB&T Capital Markets, underwriters in this offering, act as lenders and/or agents under our $372.5 million Credit Facility. As described above, we intend to use net proceeds of this offering to repay the outstanding indebtedness under this Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting—Conflicts of Interest" below.

S-13

Table of Contents

CAPITALIZATION

        The following table sets forth our capitalization:

•

on an actual basis as of June 30, 2013; and



•

on an as-adjusted basis giving effect to the sale of 4,000,000 shares of our common stock in this offering at the public offering price of $29.75 per share, less estimated underwriting discounts and offering expenses payable by us.

        This table should be read in conjunction with "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in this prospectus supplement.

S-14

Table of Contents

SELECTED FINANCIAL DATA

        The selected financial and other data below reflects the consolidated financial condition and the consolidated statement of operations of Main Street and its subsidiaries as of and for the years ended December 31, 2012, 2011, 2010, 2009 and 2008 and as of and for the six months ended June 30, 2013 and 2012. The selected financial data at December 31, 2012, 2011, 2010, 2009 and 2008 and for the years ended December 31, 2012, 2011, 2010, 2009 and 2008, have been derived from consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. The selected financial data at June 30, 2013, and for the six months ended June 30, 2013 and 2012, have been derived from unaudited financial data but, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary to present fairly the financial condition and operating results for such interim periods. Interim results as of and for the six months ended June 30, 2013 are not necessarily indicative of the results that may be expected for the year ending December 31, 2013. You should read this selected financial data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations," "Senior Securities" and the financial statements and related notes thereto in the accompanying prospectus and "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in this prospectus supplement.

S-15

Table of Contents

S-16

Table of Contents

INTERIM MANAGEMENT'S DISCUSSION AND ANALYSIS OF
FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        The following discussion should be read in conjunction with our consolidated financial statements and notes thereto contained elsewhere in this prospectus supplement.

        Statements we make in the following discussion which express a belief, expectation or intention, as well as those that are not historical fact, are forward-looking statements that are subject to risks, uncertainties and assumptions. Our actual results, performance or achievements, or industry results, could differ materially from those we express in the following discussion as a result of a variety of factors, including the risks and uncertainties we have referred to under the headings "Forward-Looking Statements" and "Supplementary Risk Factors" in this prospectus supplement and "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" in the accompanying prospectus.

ORGANIZATION

        Main Street Capital Corporation ("MSCC") was formed on March 9, 2007 for the purpose of (i) acquiring 100% of the equity interests of Main Street Mezzanine Fund, LP ("MSMF") and its general partner, Main Street Mezzanine Management, LLC ("MSMF GP"), (ii) acquiring 100% of the equity interests of Main Street Capital Partners, LLC (the "Investment Manager"), (iii) raising capital in an initial public offering, which was completed in October 2007 (the "IPO"), and (iv) thereafter operating as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSMF is licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA") and the Investment Manager acts as MSMF's manager and investment adviser. Because the Investment Manager, which employs all of the executive officers and other employees of MSCC, is wholly owned by us, we do not pay any external investment advisory fees, but instead we incur the operating costs associated with employing investment and portfolio management professionals through the Investment Manager. The IPO and related transactions discussed above were consummated in October 2007 and are collectively termed the "Formation Transactions."

        On January 7, 2010, MSCC consummated transactions (the "Exchange Offer") to exchange 1,239,695 shares of its common stock for approximately 88% of the total dollar value of the limited partner interests in Main Street Capital II, LP ("MSC II" and, together with MSMF, the "Funds"). Pursuant to the terms of the Exchange Offer, 100% of the membership interests in the general partner of MSC II, Main Street Capital II GP, LLC ("MSC II GP"), were also transferred to MSCC for no consideration. MSC II commenced operations in January 2006, is an investment fund that operates as an SBIC and is also managed by the Investment Manager. During the first quarter of 2012, MSCC exchanged 229,634 shares of its common stock to acquire all of the remaining minority ownership in the total dollar value of the MSC II limited partnership interests, including approximately 5% owned by affiliates of MSCC (the "Final MSC II Exchange"). After the completion of the Final MSC II Exchange, MSCC owns 100% of MSC II. The Exchange Offer and related transactions, including the transfer of the MSC II GP interests and the Final MSC II Exchange, are collectively termed the "Exchange Offer Transactions."

        MSCC has elected to be treated for federal income tax purposes as a regulated investment company ("RIC") under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"). As a result, MSCC generally will not pay corporate-level federal income taxes on any net ordinary income or capital gains that it distributes to its stockholders as dividends.

        MSCC has direct and indirect wholly owned subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of these entities is to hold certain investments that generate "pass through" income for tax purposes. The Investment Manager is also a direct wholly

S-17

Table of Contents

owned subsidiary that has elected to be a taxable entity. The Taxable Subsidiaries and the Investment Manager are each taxed at their normal corporate tax rates based on their taxable income.

        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our" and "Main Street" refer to MSCC and its consolidated subsidiaries, which include the Funds, the Taxable Subsidiaries and, beginning April 1, 2013, the Investment Manager.

OVERVIEW

        We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our portfolio investments are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in diverse industry sectors. We seek to partner with entrepreneurs, business owners and management teams and generally provide "one stop" financing alternatives within our LMM portfolio. We invest primarily in secured debt investments, equity investments, warrants and other securities of LMM companies based in the United States and in secured debt investments of Middle Market companies generally headquartered in the United States. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $25 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $15 million. Our private loan ("Private Loan") investments are made in businesses that are consistent with the size of companies in our LMM portfolio or our Middle Market portfolio, but are investments which have been originated through strategic relationships with other investment funds on a collaborative basis. The structure, terms and conditions for these Private Loan investments are typically consistent with the structure, terms and conditions for the loans made in our LMM portfolio or Middle Market portfolio.

        Our other portfolio ("Other Portfolio") investments primarily consist of investments which are not consistent with the typical profiles for our LMM, Middle Market or Private Loan portfolio investments, including investments which may be managed by third parties. In our Other Portfolio, we may incur indirect fees and expenses in connection with investments managed by third parties, such as investments in other investment companies or private funds.

        We seek to fill the current financing gap for LMM businesses, which, historically, have had more limited access to financing from commercial banks and other traditional sources. The underserved nature of the LMM creates the opportunity for us to meet the financing needs of LMM companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from senior secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing options, or a "one stop" financing solution. Providing customized, "one stop" financing solutions has become even more relevant to our LMM portfolio companies in the current investing environment. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Our LMM portfolio debt investments are generally secured by a first lien on the assets of the portfolio company and typically have a term of between five and seven years. We believe that our LMM investment strategy has a lower correlation to the broader debt and equity markets.

        As of June 30, 2013, we had debt and equity investments in 58 LMM portfolio companies with an aggregate fair value of approximately $555.3 million, with a total cost basis of approximately

S-18

Table of Contents

$438.7 million, and a weighted average annual effective yield on our LMM debt investments of approximately 15.1%. As of June 30, 2013 approximately 75% of our total LMM portfolio investments at cost were in the form of debt investments and approximately 94% of such debt investments at cost were secured by first priority liens on the assets of our LMM portfolio companies. At June 30, 2013, we had equity ownership in approximately 93% of our LMM portfolio companies and the average fully diluted equity ownership in those portfolio companies was approximately 33%. As of December 31, 2012, we had debt and equity investments in 56 LMM portfolio companies with an aggregate fair value of approximately $482.9 million, with a total cost basis of approximately $380.5 million, and a weighted average annual effective yield on our LMM debt investments of approximately 14.3%. As of December 31, 2012, approximately 75% of our total LMM portfolio investments at cost were in the form of debt investments and approximately 93.0% of such debt investments at cost were secured by first priority liens on the assets of our LMM portfolio companies. At December 31, 2012, we had equity ownership in approximately 93% of our LMM portfolio companies and the average fully diluted equity ownership in those portfolio companies was approximately 33%. The weighted average annual yields were computed using the effective interest rates for all debt investments at June 30, 2013 and December 31, 2012, including amortization of deferred debt origination fees and accretion of original issue discount but excluding fees payable upon repayment of the debt instruments and any debt investments on non-accrual status.

        Additionally, we pursue investments in Middle Market companies. Our Middle Market portfolio investments primarily consist of direct or secondary purchases of interest-bearing debt securities in privately held companies that are generally larger in size than the LMM companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien on the assets of the company and have an expected duration of between three and five years.

        As of June 30, 2013, we had Middle Market portfolio investments in 95 companies, collectively totaling approximately $445.2 million in fair value with a total cost basis of approximately $440.4 million. The weighted average revenue for the 95 Middle Market portfolio company investments was approximately $576.8 million as of June 30, 2013. As of June 30, 2013, substantially all of our Middle Market portfolio investments were in the form of debt investments and approximately 92% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Middle Market portfolio debt investments was approximately 7.9% as of June 30, 2013. As of December 31, 2012, we had Middle Market portfolio investments in 79 companies, collectively totaling approximately $352.0 million in fair value with a total cost basis of approximately $348.1 million. The weighted average revenue for the 79 Middle Market portfolio company investments was approximately $533.6 million as of December 31, 2012. As of December 31, 2012, substantially all of our Middle Market portfolio investments were in the form of debt investments and approximately 91% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Middle Market portfolio debt investments was approximately 8.0% as of December 31, 2012. The weighted average annual yields were computed using the effective interest rates for all debt investments at June 30, 2013 and December 31, 2012, including amortization of deferred debt origination fees and accretion of original issue discount but excluding fees payable upon repayment of the debt instruments.

        Our Private Loan portfolio investments primarily consist of investments in interest-bearing debt securities in companies that are consistent with the size of the companies included in our LMM portfolio or our Middle Market portfolio. Our Private Loan portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and seven years.

        As of June 30, 2013, we had Private Loan portfolio investments in 11 companies, collectively totaling approximately $78.9 million in fair value with a total cost basis of approximately $79.3 million.

S-19

Table of Contents

The weighted average revenue for the 11 Private Loan portfolio company investments was approximately $309.4 million as of June 30, 2013. As of June 30, 2013, 99% of our Private Loan portfolio investments were in the form of debt investments and all such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Private Loan portfolio debt investments was approximately 12.2% as of June 30, 2013. As of December 31, 2012, we had Private Loan portfolio investments in 9 companies, collectively totaling approximately $65.5 million in fair value with a total cost basis of approximately $64.9 million. The weighted average revenue for the 9 Private Loan portfolio company investments was approximately $230.5 million as of December 31, 2012. As of December 31, 2012, 99% of our Private Loan portfolio investments were in the form of debt investments and all such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average annual effective yield on our Private Loan portfolio debt investments was approximately 14.8% as of December 31, 2012. The weighted average annual yields were computed using the effective interest rates for all debt investments at June 30, 2013 and 2012, including amortization of deferred debt origination fees and accretion of original issue discount but excluding fees payable upon repayment of the debt instruments.

        As of June 30, 2013, we had Other Portfolio investments in 6 companies, collectively totaling approximately $34.6 million in fair value and approximately $32.3 million in cost basis and which comprised 3.1% of our Investment Portfolio at fair value as of June 30, 2013. As of December 31, 2012, we had Other Portfolio investments in 3 companies, collectively totaling approximately $24.1 million in both fair value and approximately $23.6 million in cost basis and which comprised 2.6% of our Investment Portfolio at fair value as of December 31, 2012.

        During the six months ended June 30, 2013, there were ten portfolio company investment transfers from the LMM and Middle Market portfolio investment categories to the Private Loan portfolio investment category totaling $69.6 million in fair value and $69.0 million in cost as of the date of transfer.

        Our portfolio investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as MSMF and MSC II are both wholly owned subsidiaries of MSCC.

        The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate the identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long-term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation will also fluctuate depending upon portfolio activity and the performance of our individual portfolio companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

        MSCC and its consolidated subsidiaries are internally managed by the Investment Manager, a wholly owned subsidiary of MSCC, which employs all of the executive officers and other employees of Main Street. Because the Investment Manager is wholly owned by MSCC, Main Street does not pay any external investment advisory fees, but instead incurs the operating costs associated with employing investment and portfolio management professionals through the Investment Manager. We believe that our internally managed structure provides us with a beneficial operating expense structure when

S-20

Table of Contents

compared to other publicly-traded and privately-held investment firms which are externally managed, and our internally managed structure allows us the opportunity to leverage our non-interest operating expenses as we grow our investment portfolio. For the three and six months ended June 30, 2013, the ratio of our total operating expenses, excluding interest expense, as a percentage of our quarterly average total assets was 1.6% and 1.6%, respectively, on an annualized basis, compared to 1.9% and 2.0%, respectively, on an annualized basis for the three and six months ended June 30, 2012 and 1.8% for the year ended December 31, 2012.

        During May 2012, MSCC and the Investment Manager executed an investment sub-advisory agreement with HMS Adviser, LP ("HMS Adviser"), which is the investment advisor to HMS Income Fund, Inc. ("HMS Income"), a non publicly-traded BDC whose registration statement on Form N-2 was declared effective by the SEC in June 2012, to provide certain investment advisory services to HMS Adviser. MSCC is initially providing such investment advisory services to HMS Adviser, but it is ultimately intended that the Investment Manager provide such services because the fees MSCC receives from such arrangement could otherwise have negative consequences on its ability to meet the source-of-income requirement necessary for it to maintain its RIC tax treatment (MSCC or the Investment Adviser, whichever is providing such investment advisory services, the "Sub-Adviser"). Certain relief must be obtained from the SEC before the Investment Manager is permitted to provide these services to HMS Adviser, which relief is being sought, but there can be no assurance that it will be obtained. Under the investment sub-advisory agreement, the Sub-Adviser is entitled to 50% of the base management fee and the incentive fees earned by HMS Adviser under its advisory agreement with HMS Income. However, the Sub-Adviser has agreed to waive all such fees from the effective date of HMS Adviser's registration statement on Form N-2 through December 31, 2013. As a result, as of June 30, 2013, the Sub-Adviser has not received any base management fee or incentive fees under the investment sub-advisory agreement and the Sub-Adviser is not due any unpaid compensation for any base management fee or incentive fees under the investment sub-advisory agreement.

CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our financial statements are prepared in accordance with generally accepted accounting principles in the United States of America ("U.S. GAAP"). For the three months ended March 31, 2013 and the three and six months ended June 30, 2012, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries. Beginning April 1, 2013, and for all periods thereafter, our consolidated financial statements also include the balance sheet and income statement accounts and other information for the Investment Manager reflected as a consolidated subsidiary (see further discussion below). The Investment Portfolio, as used herein, refers to all of our LMM portfolio investments, Middle Market portfolio investments, Private Loan portfolio investments, Other Portfolio investments and, for all periods up to and including March 31, 2013, the investment in the Investment Manager, but excludes all "Marketable securities and idle funds investments," and for all periods after March 31, 2013, the Investment Portfolio also excludes the Investment Manager. For all periods up to and including the period ending March 31, 2013, the Investment Manager was accounted for as a portfolio investment and was not consolidated with MSCC and its consolidated subsidiaries. For all periods after March 31, 2013, the Investment Manager is consolidated with MSCC and its other consolidated subsidiaries. Marketable securities and idle funds investments are classified as financial instruments and are reported separately on our Consolidated Balance Sheets and Consolidated Schedule of Investments due to the nature of such investments. Our results of operations for the three and six months ended June 30, 2013 and 2012, cash flows for the six months ended June 30, 2013 and 2012, and financial position as of June 30, 2013 and December 31, 2012, are presented on a consolidated basis. The effects of all intercompany transactions between Main Street and its consolidated subsidiaries have been eliminated in

S-21

Table of Contents

consolidation. Certain reclassifications have been made to prior period balances to conform with the current presentation, including certain investments previously included as part of the LMM portfolio or Middle Market portfolio that are now classified as part of the Private Loan portfolio and the reclassification of Investment Portfolio and Marketable securities and idle funds investment related activity from cash flows from investing activities to cash flows from operating activities.

        The accompanying unaudited consolidated financial statements of Main Street are presented in conformity with U.S. GAAP for interim financial information and pursuant to the requirements for reporting on Form 10-Q and Article 10 of Regulation S-X. Accordingly, certain disclosures accompanying annual financial statements prepared in accordance with U.S. GAAP are omitted. In the opinion of management, the unaudited consolidated financial results included herein contain all adjustments, consisting solely of normal recurring accruals, considered necessary for the fair presentation of financial statements for the interim periods included herein. The results of operations for the three and six months ended June 30, 2013 and June 30, 2012 are not necessarily indicative of the operating results to be expected for the full year. Also, the unaudited financial statements and notes should be read in conjunction with the audited financial statements and notes thereto for the year ended December 31, 2012. Financial statements prepared on a U.S. GAAP basis require management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed herein.

        Under the investment company rules and regulations pursuant to Article 6 of Regulation S-X and the Audit and Accounting Guide for Investment Companies issued by the American Institute of Certified Public Accountants (the "AICPA Guide"), we are precluded from consolidating portfolio company investments, including those in which we have a controlling interest, unless the portfolio company is another investment company. An exception to this general principle in the AICPA Guide occurs if we hold a controlling interest in an operating company that provides all or substantially all of its services directly to us, or to an investment company of ours. None of the portfolio investments made by us qualify for this exception, except as discussed below with respect to the Investment Manager. Therefore, the Investment Portfolio is carried on the balance sheet at fair value, with any adjustments to fair value recognized as "Net Change in Unrealized Appreciation (Depreciation)" on our Statement of Operations until the investment is realized, usually upon exit, resulting in any gain or loss being recognized as a "Net Realized Gain (Loss) from Investments." For all periods prior to and including March 31, 2013, the Investment Manager was accounted for as a portfolio investment and included as part of the Investment Portfolio in our consolidated financial statements. The Investment Manager was consolidated with MSCC and its other consolidated subsidiaries prospectively beginning April 1, 2013 as the controlled operating subsidiary was providing substantially all of its services directly or indirectly to Main Street or its portfolio companies.

Portfolio Investment Valuation

        The most significant determination inherent in the preparation of our consolidated financial statements is the valuation of our Investment Portfolio and the related amounts of unrealized appreciation and depreciation. As of June 30, 2013 and December 31, 2012, approximately 91% and 89%, respectively, of our total assets at each date represented our Investment Portfolio valued at fair value. We are required to report our investments at fair value. We follow the provisions of the Financial Accounting Standards Board ("FASB") Accounting Standards Codification ("Codification" or "ASC") 820, Fair Value Measurements and Disclosures ("ASC 820"). ASC 820 defines fair value, establishes a framework for measuring fair value, establishes a fair value hierarchy based on the quality of inputs used to measure fair value, and enhances disclosure requirements for fair value measurements.

S-22

Table of Contents

        Our business strategy calls for us to invest primarily in illiquid debt and equity securities issued by private, LMM companies and debt securities issued by Middle Market companies that are generally larger in size than the LMM companies. We also categorize some of our investments in LMM companies and Middle Market companies as Private Loan portfolio investments, which are typically debt securities issued by companies that are consistent in size with either the LMM companies or Middle Market companies, but are investments which have been originated through strategic relationships with other investment funds on a collaborative basis. Our portfolio also includes Other Portfolio investments which primarily consist of investments which are not consistent with the typical profiles for our LMM portfolio investments, Middle Market portfolio investments or Private Loan portfolio investments, including investments which may be managed by third parties. All of these portfolio investments may be subject to restrictions on resale.

        LMM investments and Other Portfolio investments generally have no established trading market while Middle Market securities generally have established markets that are not active. Private Loan investments may include investments which have no established trading market or have established markets that are not active. We determine in good faith the fair value of our Investment Portfolio pursuant to a valuation policy in accordance with ASC 820 and a valuation process approved by our Board of Directors and in accordance with the 1940 Act. Our valuation policy and process is intended to provide a consistent basis for determining the fair value of our portfolio.

        For LMM investments, we review external events, including private mergers, sales and acquisitions involving comparable companies, and include these events in the valuation process. For Middle Market portfolio investments, we primarily use observable inputs such as quoted prices in the valuation process. For Middle Market portfolio investments for which sufficient observable inputs are not available to determine fair value, we generally use a combination of observable inputs through obtaining third party quotes or other independent pricing and an approach similar to the income approach using a yield-to-maturity model used to value our LMM portfolio debt investments.

        For valuation purposes, "control" LMM portfolio investments are composed of debt and equity securities in companies for which we have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for our control LMM portfolio investments. As a result, for control LMM portfolio investments, we determine the fair value using a combination of market and income approaches. Under the market approach, we will typically use the enterprise value methodology to determine the fair value of these investments. The enterprise value is the fair value at which an enterprise could be sold in a transaction between two willing parties, other than through a forced or liquidation sale. Typically, private companies are bought and sold based on multiples of earnings before interest, taxes, depreciation and amortization, or EBITDA, cash flows, net income, revenues, or in limited cases, book value. There is no single methodology for estimating enterprise value. For any one portfolio company, enterprise value is generally described as a range of values from which a single estimate of enterprise value is derived. In estimating the enterprise value of a portfolio company, we analyze various factors, including the portfolio company's historical and projected financial results. We allocate the enterprise value to investments in order of the legal priority of the various components of the portfolio company's capital structure. We will also use the income approach to determine the fair value of these securities, based on projections of the discounted future free cash flows that the portfolio company or the debt security will likely generate, and which includes using a yield-to-maturity approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. The valuation approaches for our control LMM portfolio investments estimate the value of the investment if we were to sell, or exit, the investment. In addition, these valuation approaches consider the value associated with our ability to control the capital structure of the portfolio company, as well as the timing of a potential exit.

S-23

Table of Contents

        For valuation purposes, "non-control" LMM portfolio investments are composed of debt and equity securities in companies for which we do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for non-control LMM portfolio investments. For our non-control LMM portfolio investments, we use a combination of the market and income approaches to value our equity investments and the income approach to value our debt investments similar to the approaches used for our control LMM portfolio investments, and which includes using a yield-to-maturity approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. Our estimate of the expected repayment date of a LMM debt security is generally the legal maturity date of the instrument, as we generally intend to hold our LMM loans and debt securities to maturity. The yield-to-maturity analysis considers changes in leverage levels, credit quality, portfolio company performance and other factors. We will use the value determined by the yield-to-maturity analysis as the fair value for that security; however, because of our general intent to hold our loans to maturity, the fair value will not exceed the face amount of the LMM debt security. A change in the assumptions that we use to estimate the fair value of our LMM debt securities using the yield-to-maturity analysis could have a material impact on the determination of fair value. If there is deterioration in credit quality or if a LMM debt security is in workout status, we may consider other factors in determining the fair value of the LMM debt security, including the value attributable to the debt security from the enterprise value of the portfolio company or the proceeds that would most likely be received in a liquidation analysis.

        Our Middle Market portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are generally larger in size than the LMM companies included in our Investment Portfolio. For valuation purposes, all of our Middle Market portfolio investments are non-control investments for which we do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. We primarily use observable inputs to determine the fair value of these investments through obtaining third party quotes or other independent pricing, to the extent such sufficient observable inputs are available to determine fair value. For Middle Market portfolio investments for which sufficient observable inputs are not available to determine fair value, we generally use a combination of observable inputs through obtaining third party quotes or other independent pricing and an approach similar to the income approach using a yield-to-maturity model used to value our LMM portfolio debt investments.

        Our Private Loan portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are consistent with the size of companies included in our LMM portfolio or our Middle Market portfolio. For valuation purposes, all of our Private Loan portfolio investments are non-control investments for which we do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. As sufficient observable inputs to determine the fair value of these Private Loan portfolio investments through obtaining third party pricing or other independent pricing are not generally available, we generally use a combination of observable inputs through obtaining third party quotes or other independent pricing and an approach similar to the income approach using a yield-to-maturity model used to value our LMM portfolio debt investments.

        For valuation purposes, all of our Other Portfolio investments are non-control investments for which we generally do not have a controlling interest in the portfolio company or the ability to nominate a majority of the portfolio company's board of directors. Our Other Portfolio investments comprised 3.1% and 2.6%, respectively, of our Investment Portfolio at fair value as of June 30, 2013 and December 31, 2012. Similar to the LMM investment portfolio, market quotations for Other Portfolio equity investments are generally not readily available. For our Other Portfolio equity investments, we determine the fair value based on the fair value of the portfolio company as

S-24

Table of Contents

determined by independent third parties and based on our proportional ownership in the portfolio company, as well as the financial position and assessed risk of each of these portfolio investments. For Other Portfolio debt investments with observable inputs, we determine the fair value of these investments through obtaining third party quotes or other independent pricing, to the extent such sufficient observable inputs are available to determine fair value. To the extent observable inputs are not available, we value these Other Portfolio debt investments through an approach similar to the income approach using a yield-to-maturity model used to value our LMM portfolio debt investments.

        Due to the inherent uncertainty in the valuation process, our determination of fair value for our Investment Portfolio may differ materially from the values that would have been used had a ready market for the securities existed. In addition, changes in the market environment, portfolio company performance and other events that may occur over the lives of the investments may cause the gains or losses ultimately realized on these investments to be materially different than the valuations currently assigned. We determine the fair value of each individual investment and record changes in fair value as unrealized appreciation or depreciation.

Revenue Recognition

Interest and Dividend Income

        We record interest and dividend income on the accrual basis to the extent amounts are expected to be collected. Dividend income is recorded as dividends are declared or at the point an obligation exists for the portfolio company to make a distribution. In accordance with our valuation policy, we evaluate accrued interest and dividend income periodically for collectability. When a loan or debt security becomes 90 days or more past due, and if we otherwise do not expect the debtor to be able to service all of its debt or other obligations, we will generally place the loan or debt security on non-accrual status and cease recognizing interest income on that loan or debt security until the borrower has demonstrated the ability and intent to pay contractual amounts due. If a loan or debt security's status significantly improves regarding the debtor's ability to service the debt or other obligations, or if a loan or debt security is fully impaired, sold or written off, we will remove it from non-accrual status.

Fee Income

        We may periodically provide services, including structuring and advisory services, to our portfolio companies or other third parties. For services that are separately identifiable and evidence exists to substantiate fair value, income is recognized as earned, which is generally when the investment or other applicable transaction closes. Fees received in connection with debt financing transactions for services that do not meet these criteria are treated as debt origination fees and are deferred and accreted into interest income over the life of the financing.

Payment-in-Kind ("PIK") Interest and Cumulative Dividends

        We hold debt and preferred equity instruments in our Investment Portfolio that contain payment-in-kind ("PIK") interest and cumulative dividend provisions. The PIK interest, computed at the contractual rate specified in each debt agreement, is periodically added to the principal balance of the debt and is recorded as interest income. Thus, the actual collection of this interest may be deferred until the time of debt principal repayment. Cumulative dividends are recorded as dividend income, and any dividends in arrears are added to the balance of the preferred equity investment. The actual collection of these dividends in arrears may be deferred until such time as the preferred equity is redeemed. To maintain RIC tax treatment (as discussed below), these non-cash sources of income may need to be paid out to stockholders in the form of distributions, even though we may not have collected the PIK interest and cumulative dividends in cash. We will stop accruing PIK interest and cumulative dividends and will write off any accrued and uncollected interest and dividends in arrears when it is determined that such PIK interest and dividends in arrears are no longer collectible.

S-25

Table of Contents

Share-Based Compensation

        We account for our share-based compensation plans using the fair value method, as prescribed by ASC 718, Compensation—Stock Compensation. Accordingly, for restricted stock awards, we measured the grant date fair value based upon the market price of our common stock on the date of the grant and will amortize this fair value to share-based compensation expense over the requisite service period or vesting term.

Income Taxes

        MSCC has elected to be treated for federal income tax purposes as a RIC. As a RIC, MSCC generally will not pay corporate-level federal income taxes on any net ordinary income or capital gains that MSCC distributes to its stockholders as dividends. MSCC must generally distribute at least 90% of its investment company taxable income to qualify for pass-through tax treatment and maintain its RIC status. As part of maintaining RIC status, undistributed taxable income (subject to a 4% excise tax) pertaining to a given fiscal year may be distributed up to 12 months subsequent to the end of that fiscal year, provided such dividends are declared prior to the filing of the federal income tax return for the prior year.

        The Taxable Subsidiaries hold certain portfolio investments for us. The Taxable Subsidiaries are consolidated with us for financial reporting purposes, and the portfolio investments held by the Taxable Subsidiaries are included in our consolidated financial statements. The principal purpose of the Taxable Subsidiaries is to permit us to hold equity investments in portfolio companies which are "pass through" entities for tax purposes and continue to comply with the "source income" requirements contained in the RIC tax provisions of the Code. The Taxable Subsidiaries are not consolidated with us for income tax purposes and may generate income tax expense, or benefit, and the related tax assets and liabilities, as a result of their ownership of various portfolio investments. This income tax expense, or benefit, if any, and related tax assets and liabilities, are reflected in our consolidated financial statements.

        The Investment Manager has elected, for tax purposes, to be treated as a taxable entity, is not consolidated with us for income tax purposes and is taxed at normal corporate tax rates based on its taxable income, or loss, and, as a result of its activities, may generate income tax expense or benefit. The Investment Manager permits us to receive third party fees and continue to comply with the "source income" requirements contained in the RIC tax provisions. The taxable income, or loss, of the Investment Manager may differ from its book income, or loss, due to temporary book and tax timing differences, as well as permanent differences. Through March 31, 2013, the Investment Manager provided for any tax expense, or benefit, and related tax assets or liabilities, in its separate financial statements. Beginning April 1, 2013, the Investment Manager is included in our consolidated financial statements reflected as a consolidated subsidiary and any income tax expense, or benefit, and related tax assets and liabilities are reflected in our consolidated financial statements.

        The Taxable Subsidiaries and the Investment Manager use the liability method in accounting for income taxes. Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and their reported amounts in the financial statements, using statutory tax rates in effect for the year in which the temporary differences are expected to reverse. A valuation allowance is provided against deferred tax assets when it is more likely than not that some portion or all of the deferred tax asset will not be realized.

        Taxable income generally differs from net income for financial reporting purposes due to temporary and permanent differences in the recognition of income and expenses. Taxable income generally excludes net unrealized appreciation or depreciation, as investment gains or losses are not included in taxable income until they are realized.

S-26

Table of Contents

INVESTMENT PORTFOLIO COMPOSITION

        LMM portfolio investments principally consist of secured debt, equity warrants and direct equity investments in privately held, LMM companies. The LMM debt investments are primarily secured by either a first or second lien on the assets of the portfolio company, generally bear interest at fixed rates, and generally mature between five and seven years from the original investment date. In most LMM portfolio companies, we also receive nominally priced equity warrants and/or make direct equity investments, usually in connection with a debt investment.

        Middle Market portfolio investments primarily consist of direct or secondary investments in interest-bearing debt securities in companies that are generally larger in size than the LMM companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by either a first or second priority lien and typically have a term of between three and five years.

        Our Private Loan portfolio investments primarily consist of investments in interest-bearing debt securities in companies that are consistent with the size of companies included in our LMM portfolio or our Middle Market portfolio. Our Private Loan portfolio debt investments are generally secured by either a first or second priority lien on the assets of the portfolio company and typically have a term of between three and seven years.

        The following tables summarize the composition of our total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments at cost and fair value by type of investment as a percentage of the total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments as of June 30, 2013 and December 31, 2012 (this information excludes the Other Portfolio investments and the Investment Manager).

        The following tables summarize the composition of the total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments by geographic region of the United States or other countries at cost and fair value as a percentage of the total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments, as of June 30, 2013 and December 31, 2012 (this information excludes the Other Portfolio

S-27

Table of Contents

investments and the Investment Manager). The geographic composition is determined by the location of the corporate headquarters of the portfolio company.

        Our LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments are in companies conducting business in a variety of industries. The following tables summarize the composition of our total combined LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments, by industry at cost and fair value as of June 30,

S-28

Table of Contents

2013 and December 31, 2012 (this information excludes the Other Portfolio investments and the Investment Manager).

S-29

Table of Contents

        Our LMM portfolio investments, Middle Market portfolio investments and Private Loan portfolio investments carry a number of risks including, but not limited to: (1) investing in LMM, Middle Market, Private Loan and Other Portfolio companies which may have limited operating histories and financial resources; (2) holding investments that generally are not publicly traded and which may be subject to legal and other restrictions on resale; and (3) other risks common to investing in below investment grade debt and equity investments in our Investment Portfolio.

S-30

Table of Contents

PORTFOLIO ASSET QUALITY