Use these links to rapidly review the document
TABLE OF CONTENTS

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of
the Securities Exchange Act of 1934 (Amendment No.          )

Table of Contents

Main Street Capital Corporation
1300 Post Oak Boulevard, 8^th Floor
Houston, Texas 77056

March 15, 2017

Dear Stockholder:

        You are cordially invited to attend this year's Annual Meeting of Stockholders of Main Street Capital Corporation, which will be held on May 1, 2017, at Pearl Studio, Full Goods Building, 200 East Grayson, Suite 115, San Antonio, Texas 78215, commencing at 9:00 AM, local time. The notice of annual meeting and proxy statement following this letter describe the matters to be acted on at the meeting.

        If your shares are held in book-entry form on the records of American Stock Transfer & Trust Company, our transfer agent and registrar, we have enclosed a proxy card for your use. You may vote these shares by completing and returning the proxy card or, alternatively, calling a toll-free telephone number or using the Internet as described on the proxy card. If a broker or other nominee holds your shares in "street name," your broker has enclosed a voting instruction form, which you should use to vote those shares. The voting instruction form indicates whether you have the option to vote those shares by telephone or by using the Internet.

        Thank you for your support.

		Sincerely yours,
		VINCENT D. FOSTER Chairman of the Board and Chief Executive Officer

YOUR VOTE IS IMPORTANT.

        Whether or not you plan to attend the meeting, please take a few minutes now to vote your shares.

        Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting of Stockholders to Be Held on May 1, 2017.

        Our proxy statement and annual report on Form 10-K for the year ended December 31, 2016 are available on the Internet at http://mainstcapital.com under "SEC Filings" in the "Investors" section of our website.

        The following information applicable to the Annual Meeting may be found in the proxy statement and accompanying proxy card:

•

The date, time and location of the meeting;



•

A list of the matters intended to be acted on and our recommendations regarding those matters;



•

Any control/identification numbers that you need to access your proxy card; and



•

Information about attending the meeting and voting in person.

Table of Contents

Main Street Capital Corporation
1300 Post Oak Boulevard, 8^th Floor
Houston, Texas 77056

NOTICE OF 2017 ANNUAL MEETING OF STOCKHOLDERS

        The 2017 Annual Meeting of the Stockholders of Main Street Capital Corporation, a Maryland corporation, will be held at Pearl Studio, Full Goods Building, 200 East Grayson, Suite 115, San Antonio, Texas 78215, on Monday, May 1, 2017, at 9:00 AM local time, in order to:

(1)

elect our directors for a term of one year;



(2)

ratify our appointment of Grant Thornton LLP as our independent registered public accounting firm for the year ending December 31, 2017;



(3)

provide an advisory vote on executive compensation;



(4)

provide an advisory vote on the frequency of the advisory vote on executive compensation; and



(5)

transact such other business as may properly come before the meeting or any adjournment thereof.

        If you were a stockholder as of the close of business on March 3, 2017, you are entitled to vote at the meeting and at any adjournment thereof.

        Please indicate your vote as to the matters to be acted on at the meeting by following the instructions provided in the enclosed proxy card or voting instruction form, whether or not you plan on attending the meeting. If you plan to attend the meeting and wish to vote or change your vote there, please review the instructions set forth in the accompanying proxy statement under the caption "Voting Information."

        We have enclosed with this notice and proxy statement a copy of our annual report on Form 10-K for the year ended December 31, 2016.

		By Order of the Board of Directors,
		JASON B. BEAUVAIS Senior Vice President, General Counsel, Chief Compliance Officer and Secretary

Dated: March 15, 2017

Table of Contents

PROXY STATEMENT FOR 2017 ANNUAL MEETING OF STOCKHOLDERS

TABLE OF CONTENTS

Table of Contents

Main Street Capital Corporation
1300 Post Oak Boulevard, 8^th Floor
Houston, Texas 77056

PROXY STATEMENT

2017 ANNUAL MEETING OF STOCKHOLDERS

GENERAL INFORMATION

        This proxy statement and accompanying proxy card is being mailed to the stockholders of Main Street Capital Corporation ("Main Street," the "Company," "we," "us," or "our") beginning on March 15, 2017. Our Board of Directors, or the Board, is soliciting your proxy to vote your shares at our 2017 Annual Meeting of Stockholders (the "Annual Meeting") to be held on May 1, 2017 at Pearl Studio, Full Goods Building, 200 East Grayson, Suite 115, San Antonio, Texas 78215, at 9:00 AM local time. The Company will bear all expenses incurred in connection with this proxy solicitation, which we expect to conduct primarily by mail. In addition, our officers and regular employees may solicit your proxy by telephone, by facsimile transmission or in person, for which they will not be separately compensated. If your shares are held through a broker or other nominee (i.e., in "street name"), we have requested that your broker or nominee forward this proxy statement to you and obtain your voting instructions, for which the Company will reimburse them for reasonable out-of-pocket expenses.

        The SEC has adopted rules that permit companies and intermediaries, such as brokers, to satisfy the delivery requirements for proxy statements and annual reports with respect to two or more stockholders sharing the same address by delivering a single proxy statement addressed to those stockholders. This process, which is commonly referred to as "householding," potentially provides extra convenience for stockholders and cost savings for companies.

        Brokers may be householding our proxy materials by delivering a single proxy statement and annual report to multiple stockholders sharing an address, unless contrary instructions have been received from the affected stockholders. Once you have received notice from your broker that they will be householding materials to your address, householding will continue until you are notified otherwise or until you revoke your consent. If you did not respond that you did not want to participate in householding, you were deemed to have consented to the process. If at any time you no longer wish to participate in householding and would prefer to receive a separate proxy statement and annual report, or if you are receiving multiple copies of the proxy statement and annual report and wish to receive only one, please notify your broker if your shares are held in a brokerage account or us if you are a stockholder of record. You can notify us by sending a written request to: Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056, or by calling (713) 350-6000. In addition, we will promptly deliver, upon written or oral request to the address or telephone number above, a separate copy of the annual report and proxy statement to a stockholder at a shared address to which a single copy of the documents was delivered.

VOTING INFORMATION

Record Date and Who May Vote

        Our Board of Directors selected March 3, 2017 as the record date (the "Record Date") for determining stockholders entitled to vote at the Annual Meeting. This means that if you were a registered stockholder with our transfer agent and registrar, American Stock Transfer and Trust Company, on the Record Date, you may vote your shares on the matters to be considered by our stockholders at the Annual Meeting. If your shares were held in street name on that date, the broker

1

Table of Contents

or other nominee that was the record holder of your shares has the authority to vote them at the Annual Meeting in accordance with your instructions. They have forwarded to you this proxy statement seeking your instructions on how you want your shares voted.

        On the Record Date, 55,274,757 shares of our common stock were outstanding. Each outstanding share of common stock entitles its holder to one vote on each matter to be acted on at the Annual Meeting.

How to Vote

        For shares held of record, you can vote your shares in person at the Annual Meeting or vote now by giving us your proxy. You may give us your proxy by completing the enclosed proxy card and returning it in the enclosed U.S. postage-prepaid envelope, or by calling a toll-free telephone number or using the Internet as further described on the enclosed proxy card. Telephone and Internet voting procedures have been designed to verify your identity through a personal identification or control number and to confirm that your voting instructions have been properly recorded. If you vote using either of these electronic means, you will save us return mail expense.

        By giving us your proxy, you will be directing us on how to vote your shares at the Annual Meeting. Even if you plan on attending the Annual Meeting, we urge you to vote now by giving us your proxy. This will ensure that your vote is represented at the Annual Meeting. If you do attend the Annual Meeting, you can change your vote at that time, if you then desire to do so.

        If your shares are held in street name, the broker or nominee that holds your shares has the authority to vote them, absent your approval, only as to routine matters, which, in the case of the Annual Meeting, only applies to the proposal to ratify our appointment of our independent registered public accounting firm. For all other matters to be voted on at the Annual Meeting, the broker or nominee that holds your shares will need to obtain your authorization to vote those shares and has enclosed a voting instruction form with this proxy statement. In either case, they will vote your shares as you direct on their voting instruction form. You can vote by completing the enclosed voting instruction form and returning it in the enclosed U.S. postage-prepaid envelope. If you want to vote your shares in person at the Annual Meeting, you must obtain a valid proxy from your broker or nominee. You should refer to the instructions provided in the enclosed voting instruction form for further information. Additionally, the availability of telephone or Internet voting depends on the voting process used by the broker or nominee that holds your shares.

        You may receive more than one proxy statement and proxy card or voting instruction form if your shares are held through more than one account (e.g., through different brokers or nominees). Each proxy card or voting instruction form only covers those shares of common stock held in the applicable account. If you hold shares in more than one account, you will have to provide voting instructions as to all your accounts to vote all your shares.

How to Revoke or Change Your Vote

        For shares held of record, you may revoke a proxy or change your vote at any time before it is exercised by written notice to our Corporate Secretary, granting a new proxy or by voting in person at the Annual Meeting. Unless you attend the Annual Meeting and vote your shares in person, you should change your vote using the same method (by telephone, Internet or mail) that you first used to vote your shares. That way, the inspectors of election for the meeting will be able to verify your latest vote.

        For shares held in street name, you should follow the instructions in the voting instruction form provided by your broker or nominee to change your vote. If you want to change your vote as to shares

2

Table of Contents

held in street name by voting in person at the Annual Meeting, you must obtain a valid proxy from the broker or nominee that holds those shares for you.

Quorum

        The Annual Meeting will be held only if a quorum exists. The presence at the Annual Meeting, in person or by proxy, of holders of a majority of our outstanding shares of common stock as of the Record Date will constitute a quorum. If you attend the meeting or vote your shares using the enclosed proxy card or voting instruction form (including any telephone or Internet voting procedures provided), your shares will be counted toward a quorum, even if you abstain from voting on a particular matter.

Proposals to Be Voted on; Vote Required; and How Votes Are Counted

        We are asking you to vote on the following:

•

the election of all of the members of our Board of Directors;



•

the ratification of our appointment of Grant Thornton LLP as our independent registered public accounting firm for the year ending December 31, 2017;



•

an advisory vote on executive compensation; and



•

an advisory vote on the frequency of the advisory vote on executive compensation.

        Election of Directors.    Directors are elected by a majority of the votes cast at the Annual Meeting, in person or by proxy, with respect to each director in an uncontested election, such that a nominee for director will be elected to the Board of Directors if the votes cast FOR the nominee's election exceed the votes cast AGAINST such nominee's election. Abstentions and broker non-votes are not counted as votes cast for purposes of the election of directors and, therefore, will have no effect on the outcome of such election. Even if a nominee is not re-elected, he will remain in office as a director until his earlier resignation or removal. Each of the current director nominees has signed a letter of resignation that will be effective if the nominee is not re-elected at the Annual Meeting and the Board of Directors accepts his resignation following the meeting. If a nominee is not re-elected, the Board of Directors will decide whether to accept the director's resignation in accordance with the procedures listed in our Corporate Governance and Stock Ownership Guidelines, which are available at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

        Ratification of Independent Registered Public Accounting Firm.    The affirmative vote of a majority of the votes cast at the Annual Meeting, in person or by proxy, is required to ratify the appointment of Grant Thornton LLP to serve as our independent registered public accounting firm. Abstentions will not be included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote on this item.

        Advisory Vote on Executive Compensation.    The affirmative vote of a majority of the votes cast at the Annual Meeting, in person or by proxy, is required for the approval of the resolution in this proposal. Abstentions will not be included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote on this item. Even though your vote is advisory and therefore not binding on us, the Compensation Committee of our Board of Directors will review the voting results and take them into consideration when making future decisions regarding executive compensation.

        Advisory Vote on the Frequency of the Advisory Vote on Executive Compensation.    The period (every one year, two years or three years) receiving the greatest number of votes as set forth in this proposal will be deemed the choice of the frequency for the advisory vote on executive compensation that has

3

Table of Contents

been selected by our stockholders on an advisory basis. Abstentions will not be included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote on this item. Even though your vote is advisory and therefore will not be binding on us, the Compensation Committee of our Board of Directors will review the voting results and take them into consideration when making future decisions regarding the frequency of the advisory vote on executive compensation.

        We are not aware of any other matters that may be presented or acted on at the Annual Meeting. If you vote by signing and returning the enclosed proxy card or using the telephone or Internet voting procedures, the individuals named as proxies on the card may vote your shares, in their discretion, on any other matter requiring a stockholder vote that comes before the Annual Meeting.

Confidential Voting

        All voted proxies and ballots will be handled to protect your voting privacy as a stockholder. Your vote will not be disclosed except:

•

to meet any legal requirements;



•

to permit the inspectors of election to tabulate and certify your vote; or



•

to adequately respond to your written comments on your proxy card.

ELECTION OF DIRECTORS

(ITEM 1)

        Pursuant to our articles of incorporation, each member of our Board of Directors serves a one-year term, until the next annual meeting of stockholders and until his respective successor is duly qualified and elected. Currently, our Board of Directors has eight members, of whom seven are not "interested persons" of Main Street, as defined in the Investment Company Act of 1940 (the "1940 Act"). The 1940 Act requires that our Board of Directors be composed of a majority of members who are not "interested persons" of Main Street. In addition, the New York Stock Exchange Listed Company Manual requires that we maintain a majority of independent directors on the Board of Directors and provides that a director of a business development company, like us, shall be considered to be independent if he or she is not an "interested person" of the business development company, as defined in the 1940 Act.

        The term of office of all directors will expire at this year's Annual Meeting. On the recommendation of the Nominating and Corporate Governance Committee and the nomination of our Board of Directors, Messrs. Michael Appling Jr., Joseph E. Canon, Arthur L. French, J. Kevin Griffin, John E. Jackson, Brian E. Lane, Stephen B. Solcher and Vincent D. Foster will stand for reelection as directors at the Annual Meeting for a term of one year.

        Unless otherwise directed, the persons named as proxies on the enclosed proxy card intend to vote "FOR" the election of the nominees. If any nominee should become unavailable for election, the shares will be voted for such substitute nominee as may be proposed by our Board of Directors. However, we are not aware of any circumstances that would prevent any of the nominees from serving.

        Set forth below is certain information (as of March 3, 2017) with respect to the nominees for election as directors. In addition to the information presented below regarding each nominee's specific experience, qualifications, attributes and skills that led our Board of Directors to the conclusion that he should serve as a director, we also believe that all of our director nominees have a reputation for integrity, honesty and adherence to high ethical standards. They each have demonstrated business acumen and an ability to exercise sound judgment, as well as a commitment of service to Main Street and our Board of Directors. The business address of each nominee listed below is 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056.

4

Table of Contents

Nominees

5

Table of Contents

6

Table of Contents

7

Table of Contents

8

Table of Contents

        Directors are elected by a majority of the votes cast at the Annual Meeting, in person or by proxy, in an uncontested election, such that a nominee for director will be elected to the Board of Directors if the votes cast FOR the nominee's election exceed the votes cast AGAINST such nominee's election. Abstentions and broker non-votes are not counted as votes cast for purposes of the election of directors and, therefore, will have no effect on the outcome of such election. Even if a nominee is not re-elected, he will remain in office as a director until his earlier resignation or removal. Each of the current director nominees has signed a letter of resignation that will be effective if the nominee is not re-elected at the Annual Meeting and the Board of Directors accepts his resignation following the meeting. If a nominee is not re-elected, the Board of Directors will decide whether to accept the director's resignation in accordance with the procedures listed in our Corporate Governance and Stock Ownership Guidelines, which are available at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

THE BOARD OF DIRECTORS RECOMMENDS YOU VOTE "FOR" THE NOMINEES
NAMED IN THIS PROXY STATEMENT.

9

Table of Contents

CORPORATE GOVERNANCE

        We maintain a corporate governance section on our website which contains copies of the charters for the committees of our Board of Directors. The corporate governance section may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website. The corporate governance section contains the following documents, which are available in print to any stockholder who requests a copy in writing to Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056:

Audit Committee Charter
Nominating and Corporate Governance Committee Charter
Compensation Committee Charter

        In addition, our Code of Business Conduct and Ethics and our Corporate Governance and Stock Ownership Guidelines may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website and are available in print to any stockholder who requests a copy in writing. We intend to disclose any substantive amendments to, or waivers from, our Code of Business Conduct and Ethics within four business days of the waiver or amendment through a posting on our website.

Director Independence

        Our Board of Directors currently consists of eight members, seven of whom are classified under applicable listing standards of the New York Stock Exchange as "independent" directors and under Section 2(a)(19) of the 1940 Act as not "interested persons." Based on these independence standards and the recommendation of the Nominating and Corporate Governance Committee, our Board of Directors has affirmatively determined that the following directors are independent:

Michael Appling Jr.
Joseph E. Canon
Arthur L. French
J. Kevin Griffin
John E. Jackson
Brian E. Lane
Stephen B. Solcher

        Our Board of Directors considered certain portfolio investments and other transactions in which our independent directors may have had a direct or indirect interest, including the transactions, if any, described under the heading "Certain Relationships and Related Party Transactions" in evaluating each director's independence under the 1940 Act and applicable listing standards of the New York Stock Exchange, and the Board of Directors determined that no such transaction would impact the ability of any director to exercise independent judgment or impair his independence.

Communications with the Board

        Stockholders or other interested persons may send written communications to the members of our Board of Directors, addressed to Board of Directors, c/o Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056. All communications received in this manner will be delivered to one or more members of our Board of Directors.

Board Leadership Structure

        Mr. Foster currently serves as both our Chief Executive Officer and as the Chairman of our Board of Directors. As our Chief Executive Officer, Mr. Foster is an "interested person" under Section 2(a)(19) of the 1940 Act. The Board believes that the Company's Chief Executive Officer is

10

Table of Contents

currently best situated to serve as Chairman given his history with the Company, his deep knowledge of the Company's business and his extensive experience in managing private debt and equity investments in lower middle market companies and debt investments in middle market companies. The Company's independent directors bring experience, oversight and expertise from outside the Company and industry, while the Chief Executive Officer brings company-specific and industry-specific experience and expertise. The Board believes that the combined role of Chairman and Chief Executive Officer promotes strategy development and execution while facilitating effective, timely communication between management and the Board. At the present time, the independent directors feel that the combined Chairman and Chief Executive Officer responsibility is optimum for effective corporate governance.

        Our Board of Directors designated Arthur L. French as Lead Independent Director to preside at all executive sessions of non-management directors. In the Lead Independent Director's absence, the remaining non-management directors may appoint a presiding director by majority vote. The non-management directors meet in executive session without management on a regular basis. The Lead Independent Director also has the responsibility of consulting with management on Board and committee meeting agendas, acting as a liaison between management and the non-management directors, including maintaining frequent contact with the Chairman and Chief Executive Officer and facilitating collaboration and communication between the non-management directors and management. Stockholders or other interested persons may send written communications to Arthur L. French, addressed to Lead Independent Director, c/o Main Street Capital Corporation, Corporate Secretary's Office, 1300 Post Oak Blvd., 8^th Floor, Houston, Texas 77056.

        A key responsibility of the Board is to oversee the development of corporate plans and hold management accountable for the execution of major strategies. The Board believes the combined role of Chairman and Chief Executive Officer, together with the role of the Lead Independent Director, is in the best interest of our stockholders because it provides the appropriate balance between strategy development and independent oversight of management.

Board of Directors and its Committees

        Board of Directors.    Our Board of Directors met four times and acted by unanimous written consent fourteen times during 2016. All directors attended 100% of the meetings of the Board of Directors and of the committees on which they served during 2016, and all eight directors attended the 2016 Annual Meeting of Stockholders in person. Our Board of Directors expects each director to make a diligent effort to attend all Board and committee meetings, as well as each Annual Meeting of Stockholders.

        Committees.    Our Board of Directors currently has, and appoints the members of, standing Audit, Compensation and Nominating and Corporate Governance Committees. Each of those committees is

11

Table of Contents

comprised entirely of independent directors and has a written charter approved by our Board of Directors. The current members of the committees are identified in the following table.

        Audit Committee.    During the year ended December 31, 2016, the Audit Committee met four times. The Audit Committee is responsible for selecting, engaging and discharging our independent accountants, reviewing the plans, scope and results of the audit engagement with our independent accountants, approving professional services provided by our independent accountants (as well as the compensation for those services), reviewing the independence of our independent accountants and reviewing the adequacy of our internal control over financial reporting. In addition, the Audit Committee is responsible for assisting our Board of Directors with its review and approval of the determination of the fair value of our investments. Our Board of Directors has determined that each of Messrs. Appling, Griffin, Jackson and Solcher is an "Audit Committee financial expert" as defined by the Securities and Exchange Commission, or the SEC. For more information on the backgrounds of these directors, see their biographical information under "Election of Directors" above.

        Compensation Committee.    During the year ended December 31, 2016, the Compensation Committee met four times and acted by unanimous written consent twice. The Compensation Committee determines the compensation and related benefits for our executive officers including the amount of salary, bonus and stock-based compensation to be included in the compensation package for each of our executive officers. In addition, the Compensation Committee assists the Board of Directors in developing and evaluating the compensation of our non-management directors and evaluating succession planning with respect to the chief executive officer and other key executive positions. The Compensation Committee has the authority to engage the services of outside advisers, experts and others as it deems necessary to assist the committee in connection with its responsibilities. The actions of the Compensation Committee are generally reviewed and ratified by the entire Board of Directors, except the employee director does not vote with respect to his compensation.

        Nominating and Corporate Governance Committee.    During the year ended December 31, 2016, the Nominating and Corporate Governance Committee met four times. The Nominating and Corporate Governance Committee is responsible for determining criteria for service on our Board of Directors, identifying, researching and recommending to the Board of Directors director nominees for election by our stockholders, selecting nominees to fill vacancies on our Board of Directors or a committee of the Board, developing and recommending to our Board of Directors any amendments to our corporate governance principles and overseeing the self-evaluation of our Board of Directors and its committees.

Compensation Committee Interlocks and Insider Participation

        Each member of the Compensation Committee is independent for purposes of the applicable listing standards of the New York Stock Exchange. During the year ended December 31, 2016, no member of the Compensation Committee was an officer, former officer or employee of ours or had a relationship disclosable under "Certain Relationships and Related Party Transactions," except as

12

Table of Contents

disclosed therein. No interlocking relationship, as defined by the rules adopted by the SEC, existed during the year ended December 31, 2016 between any member of the Board of Directors or the Compensation Committee and an executive officer of Main Street.

Director Nomination Process

        Our Nominating and Corporate Governance Committee has determined that a candidate for election to our Board of Directors must satisfy certain general criteria, including, among other things:

•

be an individual of the highest character and integrity and have an inquiring mind, vision, a willingness to ask hard questions and the ability to work professionally with others;



•

be free of any conflict of interest that would violate any applicable law or regulation or interfere with the proper performance of the responsibilities of a director;



•

be willing and able to devote sufficient time to the affairs of our company and be diligent in fulfilling the responsibilities of a member of our Board of Directors and a member of any committee thereof (including: developing and maintaining sufficient knowledge of our company and the specialty finance industry in general; reviewing and analyzing reports and other information important to responsibilities of our Board of Directors and any committee thereof; preparing for, attending and participating in meetings of our Board of Directors and meetings of any committee thereof; and satisfying appropriate orientation and continuing education guidelines); and



•

have the capacity and desire to represent the balanced, best interests of our stockholders as a whole and not primarily a special interest group or constituency.

        The Nominating and Corporate Governance Committee seeks to identify potential director candidates who will strengthen the Board of Directors and will contribute to the overall mix of general criteria identified above. In addition to the general criteria, the Nominating and Corporate Governance Committee considers specific criteria, such as particular skills, experiences (whether in business or in other areas such as public service, academia or scientific communities), areas of expertise, specific backgrounds, and other characteristics, that should be represented on the Board of Directors to enhance its effectiveness and the effectiveness of its committees. The Nominating and Corporate Governance Committee does not have a formal policy with respect to diversity; however, the Board and the Nominating and Corporate Governance Committee believe that it is essential that the Board members represent diverse experience and viewpoints and a diverse mix of the specific criteria above. The process of identifying potential director candidates includes establishing procedures for soliciting and reviewing potential nominees from directors and for advising those who suggest nominees of the outcome of such review. The Nominating and Corporate Governance Committee also has the authority to retain and terminate any search firm used to identify director candidates.

        Any stockholder may nominate one or more persons for election as one of our directors at an annual meeting of stockholders if the stockholder complies with the notice, information and consent provisions contained in our by-laws and any other applicable law, rule or regulation regarding director nominations. When submitting a nomination to our company for consideration, a stockholder must provide certain information that would be required under applicable SEC rules, including the following minimum information for each director nominee: full name, age and address; number of any shares of our stock beneficially owned by the nominee, if any; the date such shares were acquired and the investment intent of such acquisition; whether such stockholder believes the nominee is an "interested person" of our company, as defined in 1940 Act; and all other information required to be disclosed in solicitations of proxies for election of directors in an election contest or is otherwise required, including the nominee's written consent to being named in the proxy statement as a nominee and to serving as a

13

Table of Contents

director if elected. See "Stockholders' Proposals" in this proxy statement and our by-laws for other requirements of stockholder proposals.

        The Nominating and Corporate Governance Committee will consider candidates identified through the processes described above, and will evaluate each of them, including incumbents, based on the same criteria. The Nominating and Corporate Governance Committee also takes into account the contributions of incumbent directors as Board members and the benefits to us arising from their experience on our Board of Directors. Although the Nominating and Corporate Governance Committee will consider candidates identified by stockholders, the Nominating and Corporate Governance Committee may determine not to recommend those candidates to our Board of Directors, and our Board of Directors may determine not to nominate any candidates recommended by the Nominating and Corporate Governance Committee. None of the director nominees named in this proxy statement were nominated by stockholders.

Board's Role in the Oversight of Risk Management

        Our Board of Directors as a whole has responsibility for risk oversight, with reviews of certain areas being conducted by the relevant Board committees that report on their deliberations to the full Board. The oversight responsibility of the Board and its committees is enabled by management reporting processes that are designed to provide visibility to the Board about the identification, assessment and management of critical risks and management's risk mitigation strategies. Areas of focus include competitive, economic, operational, financial (accounting, credit, liquidity and tax), legal, regulatory, compliance and other risks. The Board and its committees oversee risks associated with their respective principal areas of focus, as summarized below. Committees meet in executive session with key management personnel regularly and with representatives of outside advisors as necessary.

14

Table of Contents

COMPENSATION OF DIRECTORS

        The following table sets forth the compensation that we paid during the year ended December 31, 2016 to our non-employee directors. Directors who are also employees of Main Street or any of its subsidiaries do not receive compensation for their services as directors.

Director Compensation Table

15

Table of Contents

        The compensation for non-employee directors for 2016 was comprised of cash compensation paid to or earned by directors in connection with their service as a director. That cash compensation consisted of an annual retainer of $165,000, and an additional $40,000 retainer for the Lead Independent Director. Non-employee directors do not receive fees based on meetings attended absent circumstances that require an exceptionally high number of meetings within an annual period. We also reimburse our non-employee directors for all reasonable expenses incurred in connection with their service on our Board. The chairs of our Board committees and members of the Audit Committee received additional annual retainers for 2016 as follows:

•

the chair of the Audit Committee: $30,000;



•

members of the Audit Committee: $12,500;



•

the chair of the Compensation Committee: $17,500;



•

members of the Compensation Committee: $7,500;



•

the chair of the Nominating and Corporate Governance Committee: $12,500; and



•

members of the Nominating and Corporate Governance Committee: $5,000.

        The Non-Employee Director Plan provides a means through which we may attract and retain qualified non-employee directors to enter into and remain in service on our Board of Directors. Under the Non-Employee Director Plan, at the beginning of each one-year term of service on our Board of Directors, each non-employee director receives a number of shares equivalent to $30,000 based on the closing price of a share of our common stock on the New York Stock Exchange (or other exchange on which our shares are then listed) on the date of grant. These shares are subject to forfeiture provisions that will lapse as to an entire award at the end of the one-year term.

        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan. The 2015 Deferred Compensation Plan became effective on January 1, 2016 and replaced the Deferred Compensation Plan for Non-Employee Directors previously adopted and approved by the Board in 2013. Under the 2015 Deferred Compensation Plan, non-employee directors and certain key employees may defer receipt of some or all of their cash compensation, subject to certain limitations. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan and are also able to direct investments made on their behalf among investment alternatives permitted from time to time under the plan, including phantom Main Street stock units.

        For the beneficial ownership of our common stock by each of our directors and the dollar range value of such ownership, please see "Security Ownership of Certain Beneficial Owners and Management" beginning on page 33.

16

Table of Contents

EXECUTIVE OFFICERS

        Our executive officers serve at the discretion of our Board of Directors. The following persons serve as our executive officers or significant employees in the following capacities (information as of March 3, 2017):

        For more information on Mr. Foster, Chairman of the Board and Chief Executive Officer, see his biographical information under "Election of Directors" above.

        Dwayne L. Hyzak has served as President since 2015 and Chief Operating Officer and Senior Managing Director since 2014. Mr. Hyzak also serves as a member of our investment committee and our executive committee. Previously, he served as Chief Financial Officer and Senior Managing Director from 2011 and in other executive positions at Main Street since 2007. From 2002, Mr. Hyzak has also served as a Senior Managing Director and in other executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours. From 2000 to 2002, Mr. Hyzak was a director of integration with Quanta Services, Inc. (NYSE: PWR), which provides specialty contracting services to the power, natural gas and telecommunications industries, where he was principally focused on the company's mergers and acquisitions and corporate finance activities. Prior to joining Quanta Services, Inc., Mr. Hyzak, a certified public accountant, was a manager with Arthur Andersen in its Transaction Advisory Services group.

        Curtis L. Hartman has served as Vice Chairman since 2015 and Chief Credit Officer and Senior Managing Director since 2011. Mr. Hartman is also the chairman of our credit committee and a member of our investment committee and our executive committee. Previously, he served as Senior Vice President and in other executive positions at Main Street since 2007. From 2000, Mr. Hartman has also served as a Senior Managing Director and in other executive positions of several Main Street

17

Table of Contents

predecessor funds and entities, which are now subsidiaries of ours. Mr. Hartman also served on the Board of Directors of HMS Income Fund, Inc., a non-publicly traded business development company of which MSC Adviser I, LLC, a wholly owned subsidiary of Main Street, acts as the investment sub-adviser, from 2013 to April 2016. Mr. Hartman currently serves as an executive officer of the Small Business Investor Alliance (SBIA) and has been a member of SBIA's Board of Governors since 2011 where he previously chaired the BDC Committee. From 1999 to 2000, Mr. Hartman was a Director for Sterling City Capital, LLC, a private investment firm. Concurrently with joining Sterling City Capital, he joined United Glass Corporation, a Sterling City Capital portfolio company, as Director of Corporate Development. Prior to joining Sterling City Capital, Mr. Hartman, a certified public accountant, was a manager with PricewaterhouseCoopers LLP in its M&A/Transaction Services group and a senior auditor with Deloitte & Touche LLP in its Financial Assurance Group.

        David L. Magdol has served as Vice Chairman since 2015 and Chief Investment Officer and Senior Managing Director since 2011. Mr. Magdol is also the chairman of our investment committee and a member of our executive committee. Previously, he served as Senior Vice President and in other executive positions at Main Street since 2007. From 2002, Mr. Magdol has served as a Senior Managing Director and in other executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours. Mr. Magdol joined Main Street from the investment banking group at Lazard Freres & Co. Prior to Lazard, he managed a portfolio of private equity investments for the McMullen Group, a private investment firm/family office capitalized by Dr. John J. McMullen, the former owner of the New Jersey Devils and the Houston Astros. Mr. Magdol began his career in the structured finance services group of JP Morgan Chase.

        Rodger A. Stout has served as Executive Vice President since 2012 and is also a member of our credit committee. Previously, Mr. Stout served as Chief Accounting Officer and Secretary from 2007 to 2008, as Chief Compliance Officer and Treasurer from 2007 to 2012 and as Senior Vice President—Finance and Administration from 2008 to 2012. From 2006, Mr. Stout has served as Executive Vice President and in other executive positions of several Main Street predecessor funds and entities, which are now subsidiaries of ours. From 2000 to 2006, Mr. Stout was senior vice president and chief financial officer for FabTech Industries, Inc., which was one of the largest domestic structural steel fabricating companies. From 1985 to 2000, he was a senior financial executive for Jerold B. Katz Interests. He held numerous positions over his 15 year tenure with this national scope financial services conglomerate. Those positions included director, executive vice president, senior financial officer and investment officer. Prior to 1985, Mr. Stout was an international tax executive in the oil and gas service industry.

        Jason B. Beauvais has served as Senior Vice President, General Counsel, Chief Compliance Officer and Secretary since 2012. Previously, Mr. Beauvais served as Vice President, General Counsel and Secretary since 2008. From 2006 through 2008, Mr. Beauvais was an attorney with Occidental Petroleum Corporation (NYSE: OXY), an international oil and gas exploration and production company. Prior to joining Occidental Petroleum Corporation, Mr. Beauvais practiced corporate and securities law at Baker Botts L.L.P., where he primarily counseled companies in public issuances and private placements of debt and equity and handled a wide range of general corporate and securities matters as well as mergers and acquisitions.

        Brent D. Smith has served as Chief Financial Officer and Treasurer since November 2014 and previously as Senior Vice President—Finance since August 2014. Mr. Smith previously served as Executive Vice—President, Chief Financial Officer and Treasurer of Cal Dive International, Inc. from 2010 through June 2014 and in various finance and accounting roles at Cal Dive from 2005 through 2010. On March 3 2015, Cal Dive and certain of its subsidiaries, excluding its foreign subsidiaries, filed for voluntary protection under Chapter 11 of the Bankruptcy Code. Prior to joining Cal Dive, Mr. Smith was a manager with FTI Consulting (NYSE: FCN). Prior to that, Mr. Smith, a certified public accountant, was employed as a senior auditor at Arthur Andersen LLP.

18

Table of Contents

        Nicholas T. Meserve has served as Managing Director on our middle market investment team since 2012. Mr. Meserve has also served on the Board of Directors of HMS Income Fund, Inc., a non-publicly traded business development company of which MSC Advisor I, LLC, a wholly owned subsidiary of Main Street, acts as the investment sub-advisor, since April 2016. Previously, from 2004 until 2012, Mr. Meserve worked at Highland Capital Management, LP, a large alternative credit manager, and certain of its affiliates, where he managed a portfolio of senior loans and high yield bonds across a diverse set of industries. Prior to Highland, he was a Credit Analyst at JP Morgan Chase & Co.

        K. Colton Braud, III has served as a Managing Director on our lower middle market team since January 2017 and has been with the firm in Associate to Director roles since 2012. Prior to joining Main Street, Mr. Braud spent two years as an Associate at Wellspring Capital Management, a middle market private equity firm based in New York. While at Wellspring, Mr. Braud's responsibilities included evaluating leveraged buyout opportunities, conducting due diligence across a wide array of industries and portfolio management. Prior to Wellspring, Mr. Braud served as an Analyst at J.P. Morgan Securities Inc. in its Financial Sponsor Group.

        Shannon D. Martin has served as Vice President, Chief Accounting Officer and Assistant Treasurer since 2012. From 2006 to 2012, Mr. Martin worked as an independent consultant and performed financial advisory services for several clients, including functioning as acting Chief Accounting Officer from 2008 to 2011 for EquaTerra, Inc. From 1999 to 2006, Mr. Martin was a director of accounting integration and audit with Quanta Services, Inc. (NYSE: PWR), which provides specialty contracting services to the power, natural gas and telecommunications industries, where he focused on the development of integrated accounting, business and information system processes and the company's acquisition and integration strategies. From 1992 to 1999, Mr. Martin, a certified public accountant, worked at Arthur Andersen as a manager in the Commercial Services group.

        Katherine S. Silva, a certified public accountant, has served as Vice President since 2015, with responsibility for managing several administrative functions, and Assistant Treasurer since 2010. She also serves as special assistant to Mr. Foster. Ms. Silva has worked at Main Street since 2005 and holds a Bachelor of Arts in Journalism from the University of Georgia.

COMPENSATION DISCUSSION AND ANALYSIS

        The following Compensation Discussion and Analysis, or CD&A, provides information relating to the 2016 compensation of Main Street's Named Executive Officers, or NEOs, for 2016, who were:

•

Vincent D. Foster, Chairman and Chief Executive Officer;



•

Dwayne L. Hyzak, President, Chief Operating Officer and Senior Managing Director;



•

Curtis L. Hartman, Vice Chairman, Chief Credit Officer and Senior Managing Director;



•

David L. Magdol, Vice Chairman, Chief Investment Officer and Senior Managing Director; and



•

Brent D. Smith, Chief Financial Officer and Treasurer.

Compensation Philosophy and Objectives

        The Main Street compensation system was developed by the Compensation Committee and approved by all independent directors. The system is designed to attract and retain key executives, motivate them to achieve the Company's business objectives and reward them for performance while aligning management's interests with those of the Company's stockholders. The structure of Main

19

Table of Contents

Street's incentive compensation programs is formulated to encourage and reward the following, among other things:

•

achievement of income and capital gains to sustain and grow the Company's dividend payments;



•

maintenance of liquidity and capital flexibility to accomplish the Company's business objectives, including the preservation of investor capital;



•

attainment of superior risk-adjusted returns on the Company's investment portfolio; and



•

professional development and growth of individual executives, the management team and other employees.

        The Compensation Committee has the primary authority to establish compensation for the NEOs and other key employees and administers all executive compensation arrangements and policies. Main Street's Chief Executive Officer assists the Committee by providing recommendations regarding the compensation of NEOs and other key employees, excluding himself. The Committee exercises its discretion by modifying or accepting these recommendations. The Chief Executive Officer routinely attends a portion of the Committee meetings. However, the Committee often meets in executive session without the Chief Executive Officer or other members of management when discussing compensation matters and on other occasions as determined by the Committee.

        The compensation packages for Main Street NEOs and other key employees are structured to reflect the Compensation Committee's commitment to corporate governance best practices and performance-oriented executive compensation. Specifically, the Compensation Committee has implemented the following practices for NEOs and other key employees:

•

no employment agreements;



•

no cash severance benefits;



•

no supplemental defined benefit pensions;



•

no perquisite allowances; and



•

no tax gross-up payments.

        The Compensation Committee takes into account competitive market practices with respect to the salaries and total direct compensation of the NEOs and other key employees. Members of the Committee consider market practices by reviewing public and non-public information for executives at comparable companies and funds. The Committee also has the authority to utilize compensation consultants to better understand competitive pay practices and has retained such expertise in the past.

Independent Compensation Consultant

        The Compensation Committee has from time to time engaged Deloitte Consulting LLP ("Deloitte") as an independent compensation consultant to assist the Committee and provide advice on a variety of compensation matters relating to NEO, other key employee and independent director compensation, incentive compensation plans and compensation trends, regulatory matters and compensation planning best practices. The compensation consultant was hired by and reports directly to the Compensation Committee. Although the compensation consultant may work directly with management on behalf of the Compensation Committee, any such work is under the control and supervision of the Compensation Committee. The total amount of fees paid or to be paid to Deloitte for compensation consulting services during fiscal 2016 was approximately $30,959.

        During fiscal 2016, the Company's management also retained Deloitte and its affiliates to provide certain other services to the Company. These other services included (i) tax services and other tax-related services and (ii) portfolio valuation consulting services. The total amount paid or to be paid

20

Table of Contents

for such services (excluding the services as consultant to the Compensation Committee as discussed above) to Deloitte and its affiliates during fiscal 2016 was approximately $909,734. Deloitte was engaged directly by management to provide these other services and, accordingly, Deloitte's engagement for these other services was not formally approved by the Board of Directors or by the Compensation Committee. The Compensation Committee believes that, given their nature and scope, these additional services did not raise a conflict of interest and did not impair Deloitte's ability to provide independent advice to the Compensation Committee concerning executive compensation matters. In making this determination, the Compensation Committee considered, among other things, the following factors when selecting Deloitte to provide compensation consulting services: (i) the types of non-compensation services provided by Deloitte, (ii) the amount of fees for such non-compensation services, noting in particular that such fees are negligible when considered in the context of Deloitte's total revenues for the period, (iii) Deloitte's policies and procedures concerning conflicts of interest, (iv) Deloitte's representatives who advise the Compensation Committee do not provide any non-compensation related services to the Company, (v) there are no other business or personal relationships between the Company's management or members of the Compensation Committee, on the one hand, and any Deloitte representatives who provide compensation consulting services to the Company, on the other hand, and (vi) neither Deloitte nor any of the Deloitte representatives who provide compensation services to the Company own any common stock or other securities of the Company.

Assessment of Market Data

        In assessing the competitiveness of executive compensation levels, the Compensation Committee analyzes market data of certain companies, including internally managed business development companies, or BDCs, private equity firms and other asset management and financial services companies. This analysis focuses on key elements of compensation practices in general, and more specifically, the compensation practices at companies and funds reasonably comparable in asset size, typical investment size and type, market capitalization and general business scope as compared to the Company.

        As regards other internally managed BDCs like Main Street, the Compensation Committee considers the compensation practices and policies pertaining to executive officers as detailed in their company's respective proxies, research analysts' reports and other publicly available information. However, there are relatively few internally managed BDCs and none that are directly comparable to the Company as regards business strategies, assets under management, typical investment size and type and market capitalization. Moreover, regarding the compensation and retention of executive talent, the Company also competes with private equity funds, mezzanine debt funds, hedge funds and other types of specialized investment funds. Since these funds are generally private companies that are not required to publicly disclose their executive compensation practices and policies, the Committee relies on third party compensation surveys as well as other available information to compare compensation practices and policies.

        Items taken into account from comparable companies and funds include, but are not necessarily limited to, base compensation, bonus compensation, stock option awards, restricted stock awards, carried interest and other compensation. In addition to actual levels of cash and equity related compensation, the Compensation Committee also considers other approaches comparable companies are taking with regard to overall executive compensation practices. Such items include, but are not necessarily limited to, the use of employment agreements for certain employees, the mix of cash and equity compensation, the use of third party compensation consultants and certain corporate and executive performance measures that are established to achieve longer term total return for stockholders. Finally, in addition to analyzing comparable companies and funds, the Committee also

21

Table of Contents

evaluates the relative cost structure of the Company as compared to the entire BDC sector, including internally and externally managed BDCs, as well as other private funds.

Assessment of Company Performance

        The Compensation Committee believes that sustained financial performance coupled with consistent stockholders' returns as well as proportional employee compensation are essential components for Main Street's long-term business success. Main Street typically makes three to seven year investments in its portfolio companies. However, the Company's business plan involves taking on investment risks over a range of time periods. Accordingly, much emphasis is focused on maintaining the stability of net asset values as well as the continuity of earnings to pass through to stockholders in the form of recurring dividends. The quality of the earnings supporting the dividends as well as the maintenance and growth of dividends are key metrics in the Committee's assessment of financial performance.

        Main Street's primary strategy is to generate current income from debt investments and to realize capital gains from equity-related investments. This income supports the payment of dividends to stockholders. The recurring payment of dividends requires a methodical investment acquisition approach and active monitoring and management of the investment portfolio over time. A meaningful part of the Company's employee base is dedicated to the maintenance of asset values and expansion of this recurring income to sustain and grow dividends. The Committee believes that stability of the management team is critical to achieving successful implementation of the Company's strategies. Further, the Committee, in establishing and assessing executive salary and performance incentives, is more focused on Main Street results as compared to its business objectives rather than the performance of Main Street relative to other comparable companies or industry metrics.

Executive Compensation Components

        For 2016, the components of Main Street's direct compensation program for NEOs included:

•

base salary;



•

annual cash bonuses;



•

long-term compensation pursuant to the 2015 Equity and Incentive Plan; and



•

other benefits.

        The Compensation Committee designs each NEO's direct compensation package to appropriately reward the NEO for his or her contribution to the Company. The judgment and experience of the Committee are weighed with individual and Company performance metrics and consultation with the Chief Executive Officer (except with respect to himself) to determine the appropriate mix of compensation for each individual. The Compensation Committee does not target a specific level of compensation relative to market practice, and only uses such data as a reference point when establishing compensation levels for NEOs. Cash compensation consisting of base salary and discretionary bonuses tied to achievement of individual performance goals that are reviewed and approved by the Committee, as well as corporate objectives, are intended to motivate NEOs to remain with the Company and work to achieve expected business objectives. Stock-based compensation is awarded based on performance expectations approved by the Committee for each NEO. The blend of short-term and long-term compensation may be adjusted from time to time to balance the Committee's views regarding the benefits of current cash compensation and appropriate retention incentives.

22

Table of Contents

Base Salary

        Base salary is used to recognize the experience, skills, knowledge and responsibilities required of the NEOs in their roles. In connection with establishing the base salary of each NEO, the Compensation Committee and management consider a number of factors, including the seniority and experience level of the individual, the functional responsibilities of the position, the experience level of the individual, the Company's ability to replace the executive, the past base salary of the individual and the relative number of well-qualified candidates available in the area. In addition, the Committee considers publicly available information regarding the base salaries paid to similarly situated executive officers and other competitive market practices.

        The salaries of the NEOs are reviewed on an annual basis, as well as at the time of promotion or any substantial change in responsibilities. The key factors in determining increases in salary level are relative performance and competitive pressures.

Annual Cash Bonuses

        Annual cash bonuses are intended to reward individual performance on an annual basis and can therefore be variable from year to year. Cash bonus awards for the NEOs are determined by the Compensation Committee on a discretionary basis based on performance criteria, particularly the company's dividend performance as well as corporate and individual performance goals and other measures established by the Committee with the Chief Executive Officer's input (except with respect to his own performance criteria). Should actual performance exceed expected performance criteria, the Committee may adjust individual cash bonuses to take such superior performance into account.

Long-Term Incentive Awards

        Main Street's Board of Directors and stockholders approved the 2015 Equity and Incentive Plan in May 2015 to provide stock-based awards as long-term incentive compensation to employees, including the NEOs. The Company uses stock-based awards to (i) attract and retain key employees, (ii) motivate employees by means of performance-related incentives to achieve long-range performance goals, (iii) enable employees to participate in the Company's long-term growth in value and (iv) link employees' compensation to the long-term interests of stockholders. At the time of each award, the Compensation Committee will determine the terms of the award, including any performance period (or periods) and any performance objectives relating to vesting of the award. Prior to the adoption of the 2015 Equity and Incentive Plan, stock-based awards to employees were made under and are governed by the 2008 Equity Incentive Plan. Terms of the 2008 Equity Incentive Plan are substantially similar to the 2015 Equity and Incentive Plan. After adoption of the 2015 Equity and Incentive Plan no further awards have or will be granted under the 2008 Equity Incentive Plan.

        Restricted Stock.    Main Street has received exemptive relief from the SEC that permits the Company to grant restricted stock in exchange for or in recognition of services by its executive officers and employees. Pursuant to the 2015 Equity and Incentive Plan, the Compensation Committee may award shares of restricted stock to plan participants in such amounts and on such terms as the Committee determines in its sole discretion, provided that such awards are consistent with the conditions set forth in the SEC's exemptive order. Each restricted stock grant will be for a fixed number of shares as set forth in an award agreement between the grantee and Main Street. Award agreements will set forth time and/or performance vesting schedules and other appropriate terms and/or restrictions with respect to awards, including rights to dividends and voting rights. Beginning in 2015, the Committee awarded restricted stock awards to employees, including NEOs, which vest in equal increments over a three year time frame based on continued service during the vesting period. The Committee's previous practice had been to award restricted stock to employees which vested over

23

Table of Contents

a four year time frame. The change to the vesting period was made to be more closely aligned with comparable companies.

        Options.    The Compensation Committee may also grant stock options to purchase Main Street's common stock (including incentive stock options and nonqualified stock options). The Committee expects that any options granted will represent a fixed number of shares of common stock, will have an exercise price equal to the fair market value of common stock on the date of grant, and will be exercisable, or "vested," at some later time after grant. Certain stock options may provide for vesting based on the grantee remaining employed by Main Street for a time certain and/or the grantee and/or the Company attaining specified performance criteria. To date, the Committee has not granted stock options to any NEO.

Other Benefits

        Main Street's NEOs participate in the same benefit plans and programs as the Company's other employees, including comprehensive medical, dental and vision insurance, short term and long term disability insurance and life insurance.

        Main Street maintains a 401(k) plan for all full-time employees who are at least 21 years of age through which the Company makes non-discretionary matching contributions to each participant's plan account on the participant's behalf. For each participating employee, the Company's contribution is a 100% match of the employee's contributions up to a 3% contribution level and a 50% match of the employee's contributions from a 3% to a 6% contribution level, with a maximum annual regular matching contribution of $11,925 during 2016. All contributions to the plan, including those made by the Company, vest immediately. The Board of Directors may also, at its sole discretion, provide that the Company will make additional contributions to employee 401(k) plan accounts, which would also vest immediately.

        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan to allow non-employee directors and certain key employees, including each of the NEOs, to defer receipt of some or all of their cash compensation, subject to certain limitations. Although not currently anticipated and subject to prior Compensation Committee approval, discretionary employer contributions are also permitted to the 2015 Deferred Compensation Plan. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan and are also able to direct investments made on their behalf among investment alternatives permitted from time to time under the plan, including phantom Main Street stock units. The 2015 Deferred Compensation Plan became effective on January 1, 2016.

Perquisites

        The Company provides no other material benefits, perquisites or retirement benefits to the NEOs.

Potential Payments Upon Change in Control or Termination of Employment

        Unless the terms of an award provide otherwise, in the event of a specified transaction involving a "change in control" (as defined in the 2015 Equity and Incentive Plan) in which there is an acquiring or surviving entity, the Board of Directors may provide for the assumption of some or all outstanding awards, or for the grant of substitute awards, by the acquirer or survivor. In the event no such assumption or substitution occurs, each stock-based award, subject to its terms, will become fully vested or exercisable prior to the change in control on a basis that gives the holder of the award a reasonable opportunity, as determined by the Board of Directors, to participate as a stockholder in the change in control following vesting or exercise. The award will terminate upon consummation of the change in control.

24

Table of Contents

        Transactions involving a "change in control" under the 2015 Equity and Incentive Plan include the following, other than where Main Street's stockholders continue to have substantially the same proportionate ownership in an entity which owns substantially all of Main Street's assets immediately following such transaction:

•

a single person or entity or group of persons and/or entities, other than Main Street, any of its employee benefit plans, a company owned by Main Street's stockholders in substantially the same proportions as their ownership in Main Street or an underwriter temporarily holding securities pursuant to an offering by Main Street, becomes the beneficial owner of more than 30% of the combined voting power of Main Street's voting securities then outstanding;



•

a change in the membership of the Board of Directors such that the individuals who, as of the effective date of the 2015 Equity and Incentive Plan, constitute the Board of Directors (the "Continuing Directors"), and any new director whose election or nomination to the Board of Directors was approved by a vote of at least a majority of the Continuing Directors, cease to constitute at least a majority of the Board;



•

a merger, reorganization or business combination of Main Street or one of its subsidiaries with or into any other entity, other than where the holders of Main Street's voting securities outstanding immediately before such transaction would represent immediately thereafter more than a majority of the combined voting power of the voting securities of Main Street or the surviving entity or the parent of such surviving entity;



•

a sale or disposition of all or substantially all of Main Street's assets, other than where the holders of Main Street's voting securities outstanding immediately before such transaction hold securities immediately thereafter which represent more than a majority of the combined voting power of the voting securities of the acquirer or the parent of such acquirer of such assets; or



•

Main Street's stockholders approve a plan of complete liquidation or dissolution of Main Street.

        Our restricted stock awards also provide that upon a participant's death, disability, involuntary termination without cause or voluntary termination with good reason (each as defined in the award agreement), the unvested shares of restricted stock will fully vest. The number of shares and value of unvested restricted stock for each NEO as of December 31, 2016 that would have vested under the acceleration scenarios described above is shown under "Compensation of Executive Officers—Outstanding Equity Awards at Fiscal Year-End."

        In addition, NEOs who participate in the 2015 Deferred Compensation Plan could receive a distribution of their balances in that plan in connection with their death, disability or termination of employment, depending on their distribution elections under the plan. The aggregate balance in the 2015 Deferred Compensation Plan of each NEO as of December 31, 2016 is shown under "Compensation of Executive Officers—Nonqualified Deferred Compensation."

        Other than the accelerated vesting of restricted stock and amounts due under the 2015 Deferred Compensation Plan, we would not incur any other payment obligations to our NEOs in the event of a change in control or any of the aforementioned causes of termination of employment.

1940 Act Restrictions on Company Performance Based Compensation

        The 1940 Act provides that a BDC such as Main Street may maintain either an equity incentive plan or a "profit-sharing plan", but not both, for its NEOs and other employees. The Compensation Committee believes that equity incentives strongly align the interests of NEOs and employees with those of the Company's stockholders. Accordingly, Main Street has adopted and maintained equity incentive plans for its NEOs and employees since 2008. As a result, the 1940 Act prohibits Main Street from having a "profit-sharing plan."

25

Table of Contents

        The term "profit-sharing plan" is defined very broadly in the 1940 Act but in this context is generally viewed as referring to incentive and other compensation being directly tied to a company's gross or net income or any other indicia of the company's overall financial performance, such as realized gains or losses and unrealized appreciation or depreciation on investments. In this regard, the SEC has indicated that a compensation program possesses profit-sharing characteristics if a company is obligated to make payments under the program based on company performance metrics.

        Due to these restrictions imposed by the 1940 Act, the Compensation Committee is not permitted to use nondiscretionary or formulaic Company performance goals or criteria to determine executive incentive compensation. Instead, the committee considers overall company performance along with other factors, including individual performance criteria, and uses its discretion in determining the appropriate compensation for NEOs and other key employees. The Compensation Committee's objective is to work within the 1940 Act regulatory framework to establish appropriate compensation levels, maintain pay-for-performance alignment and implement compensation best practices.

Tax Deductibility of Compensation

        Section 162(m) of the Internal Revenue Code of 1986 generally disallows a deduction to public companies to the extent of excess annual compensation over $1 million paid to certain executive officers, except for qualified performance-based compensation. Main Street's general policy, where consistent with business objectives, is to preserve the deductibility of executive officer compensation. However, the Compensation Committee may authorize amounts and forms of compensation that might not be deductible if the Committee deems such to be in the best interests of Main Street and its stockholders.

Stockholder Advisory Vote on Executive Compensation

        At our 2014 Annual Meeting of Stockholders, our stockholders provided an advisory vote with 93% of the votes cast approving our compensation philosophy, policies and procedures and the 2013 fiscal year compensation of our NEOs (the "Advisory Vote"). Subsequently, the Compensation Committee considered the results of the Advisory Vote in determining compensation policies and decisions of the Company. The Advisory Vote affected the Company's executive compensation decisions and policies by reaffirming the Company's compensation philosophies, and the Compensation Committee will continue to use these philosophies and past practice in determining future compensation decisions.

2016 Compensation Determination

        The Compensation Committee analyzed the competitiveness of the components of compensation described above on both an individual and aggregate basis. The Committee believes that the total compensation paid to the NEOs for the fiscal year ended December 31, 2016, is consistent with the overall objectives of Main Street's executive compensation program.

Base Salary

        The Compensation Committee annually reviews the base salary of each executive officer, including each NEO, and determines whether or not to increase it in its sole discretion. Increases to base salary can be awarded to recognize, among other things, relative performance, relative cost of living and competitive pressures.

        In 2016, the Compensation Committee approved base salary increases for each NEO in recognition each NEO's and the Company's performance for the year and also to more closely align each NEO's compensation with similar executive officers of comparable companies.

26

Table of Contents

        The amount of annual base salary paid to each NEO for 2016 is presented under the caption entitled "Compensation of Executive Officers—Summary Compensation Table." The Committee believes that the salary changes and resulting base salaries were competitive in the market place and appropriate for Main Street executives as a key component of an overall compensation package.

Annual Cash Incentive Bonus

        Cash bonuses are determined annually by the Compensation Committee and are based on individual and corporate performance objectives coupled with Committee discretion as appropriate. The 2016 performance criteria used for determining the cash bonuses for NEOs included, among other things, the following:

•

Achievement of corporate objectives, particularly those related to the maintenance and growth of dividends and preservation of capital through maintenance and growth of net asset value per share;



•

Individual performance and achievement of individual goals, as well as the contribution to corporate objectives;



•

Maintaining liquidity and capital flexibility to accomplish the Company's business objectives;



•

Maintaining the highest ethical standards, internal controls and adherence to regulatory requirements; and



•

Appropriate and planned development of personnel.

        The Company paid cash bonuses to NEOs for 2016 performance in recognition of Main Street's strong financial results, including total stockholder return of 37.4%. Each individual NEO contributed significantly to the Company's performance. Major achievements considered by the Compensation Committee included increased distributable net investment income per share, increased regular monthly dividends per share, total stockholder return versus comparable companies and the market in general, increased net asset value per share, growth of the investment portfolio, continuation of the Company's low total operating cost structure in comparison to peer organizations, maintaining an investment grade rating from Standard & Poor's Ratings Services, low employee turnover, expansion of the Company's third party asset management business and development of talented personnel. The Compensation Committee did not weight these achievements and used discretion in determining the cash bonus amount allocated to each executive. In summary, the performance of each individual NEO and the management team overall was at a consistent high level in 2016 resulting in excellent financial results.

        The amount of cash bonus paid to each NEO for 2016 is presented under the caption entitled "Compensation of Executive Officers—Summary Compensation Table." The Committee believes that these cash bonus awards are individually appropriate based on 2016 performance. Such bonuses comprise a key component of the Company's overall compensation program.

Long-Term Incentive Awards

        The Company granted restricted shares to our NEOs in 2016 to recognize individual contributions to corporate strategic priorities and to the long-term performance of the Company. Other objectives of restricted stock awards were to assist with retention, align NEO interests with stockholders' and to provide competitive total direct compensation. Contributions to the future success of the Company include expanded roles of NEOs within the Company, recruitment and development of personnel, advancement of strategic initiatives with benefits beyond the current year, development of appropriate capital structure alternatives and enhancement of the Company's reputation with key constituents.

        The amount of restricted shares granted to each NEO in 2016 is presented under the caption entitled "Compensation of Executive Officers—Grants of Plan-Based Awards." The Committee is

27

Table of Contents

currently assessing the potential for long-term incentive compensation through grants of restricted shares to our NEOs for 2017, which are expected to be awarded in April 2017.

COMPENSATION COMMITTEE REPORT

        We have reviewed and discussed the Compensation Discussion and Analysis included in this proxy statement with Main Street's management and, based on our review and discussions, we recommended to the Board of Directors of Main Street that the Compensation Discussion and Analysis be included in this proxy statement.

28

Table of Contents

COMPENSATION OF EXECUTIVE OFFICERS

        The following table summarizes the compensation of our Named Executive Officers, or NEOs, for the fiscal year ended December 31, 2016.

Summary Compensation Table

29

Table of Contents

Grants of Plan-Based Awards

        The following table sets forth information regarding restricted stock awards granted to our NEOs in fiscal 2016:

Outstanding Equity Awards at Fiscal Year-End

        The following table sets forth the awards of restricted stock for which forfeiture provisions have not lapsed and remain outstanding at December 31, 2016:

30

Table of Contents

Equity Awards Vested in Fiscal Year

        The following table sets forth information regarding shares of restricted stock for which forfeiture restrictions lapsed during the fiscal year ended December 31, 2016:

Nonqualified Deferred Compensation

        The following table sets forth information regarding the activity during the fiscal year ended December 31, 2016 related to the accounts of our NEOs under the 2015 Deferred Compensation Plan:

31

Table of Contents

        In November 2015, our Board of Directors approved and adopted the 2015 Deferred Compensation Plan, an unfunded, nonqualified deferred compensation plan, to allow non-employee directors and certain key employees, including each of the NEOs, to defer receipt of some or all of their cash compensation, subject to certain limitations. Pursuant to the 2015 Deferred Compensation Plan, executives may contribute on a pre-tax basis up to 100% of their salary, bonus and dividends paid on shares of unvested Company stock. Although not currently anticipated and subject to prior Compensation Committee approval, discretionary employer contributions are also permitted to the 2015 Deferred Compensation Plan. Individuals participating in the 2015 Deferred Compensation Plan receive distributions of their respective balances based on predetermined payout schedules or other events as defined by the plan. Amounts deferred under the plan earn a return based on the returns on certain investment alternatives permitted under the plan, including phantom Main Street stock units, as designated by the participant. The 2015 Deferred Compensation Plan became effective on January 1, 2016.

Potential Payments Upon Change in Control or Termination of Employment

        As described in "Compensation Discussion and Analysis," our restricted stock awards to employees, including NEOs, provide that upon certain transactions involving a change in control, or upon a participant's death, disability, involuntary termination without cause or voluntary termination with good reason (each as defined in the award agreement), the unvested shares of restricted stock will fully vest. The number of shares and value of unvested restricted stock for each NEO as of December 31, 2016 that would have vested under the acceleration scenarios described above is shown under the heading "—Outstanding Equity Awards at Fiscal Year-End."

        In addition, NEOs who participate in the 2015 Deferred Compensation Plan could receive a distribution of their balances in that plan in connection with their death, disability or termination of employment, depending on their distribution elections under the plan. The aggregate balance in the 2015 Deferred Compensation Plan of each NEO as of December 31, 2016 is shown under the heading "—Nonqualified Deferred Compensation."

        Other than the accelerated vesting of restricted stock and amounts due under the 2015 Deferred Compensation Plan, we would not incur any other payment obligations to our NEOs in the event of a change in control or any of the aforementioned causes of termination of employment.

Risk Management and Compensation Policies and Practices

        We believe that risks arising from our compensation policies and practices for our employees are not reasonably likely to have a material adverse effect on the Company. In addition, the Compensation Committee believes that the mix and design of the elements of executive compensation do not encourage management to assume excessive risks.

        The Compensation Committee has reviewed the elements of executive compensation to determine whether any portion of executive compensation encouraged excessive risk taking and concluded:

•

compensation is allocated among base salary and short and long-term compensation opportunities in such a way as to not encourage excessive risk-taking;



•

significant weighting towards long-term incentive compensation discourages short-term risk taking;



•

goals are appropriately set to avoid targets that, if not achieved, result in a large percentage loss of compensation; and

32

Table of Contents

•

multi-year vesting of our equity awards and share ownership guidelines properly account for the time horizon of risk.

        Furthermore, as described in "Compensation Discussion and Analysis," compensation decisions include subjective considerations, which restrain the influence of formulae or objective factors on excessive risk taking.

SECURITY OWNERSHIP OF CERTAIN BENEFICIAL OWNERS AND MANAGEMENT

        The following table sets forth information with respect to the beneficial ownership of our common stock by:

•

each person known to us to beneficially own more than five percent of the outstanding shares of our common stock;



•

each of our directors and executive officers; and



•

all of our directors and executive officers as a group.

        Beneficial ownership is determined in accordance with the rules of the SEC and includes voting or investment power with respect to the securities. There is no common stock subject to options that are currently exercisable or exercisable within 60 days of March 3, 2017. Percentage of beneficial ownership is based on 55,274,757 shares of common stock outstanding as of March 3, 2017.

        Unless otherwise indicated, to our knowledge, each stockholder listed below has sole voting and investment power with respect to the shares beneficially owned by the stockholder, and maintains an address c/o Main Street Capital Corporation. Our address is 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056.

33

Table of Contents

        The Board of Directors has established stock ownership guidelines pursuant to which independent directors and certain key employees, including each executive officer listed in the table above, are required to achieve and maintain minimum levels of stock ownership. Our Corporate Governance and Stock Ownership Guidelines may be found at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

        Our insider trading policy prohibits our directors, officers and employees from holding shares of our common stock or other securities issued by us in a margin account, hedging any such securities or pledging any such securities as collateral for a loan except in limited cases with the pre-approval of our chief compliance officer.

34

Table of Contents

        The following table sets forth, as of March 3, 2017, the dollar range of our equity securities that is beneficially owned by each of our directors.

35

Table of Contents

AUDIT COMMITTEE REPORT

        The Audit Committee is appointed by the Board of Directors to review Main Street's financial matters. Each member of the Audit Committee meets the independence requirements established by the 1940 Act and under the applicable listing standards of the New York Stock Exchange. As the Audit Committee, we are responsible for the selection, engagement, compensation, retention and oversight of Main Street's independent registered public accounting firm. We are also responsible for recommending to the Board of Directors that Main Street's audited financial statements be included in its annual report on Form 10-K for the fiscal year.

        In making our recommendation that Main Street's financial statements be included in its annual report on Form 10-K for the year ended December 31, 2016, we have taken the following steps:

•

We discussed with Grant Thornton LLP, or Grant Thornton, Main Street's independent registered public accounting firm for the year ended December 31, 2016, those matters required to be discussed by Auditing Standard No. 16, Communications with Audit Committees, and Auditing Standards No. 18, Related Parties, issued by the Public Company Accounting Oversight Board, including information regarding the scope and results of the audit. These communications and discussions are intended to assist us in overseeing the financial reporting and disclosure process.



•

We conducted periodic executive sessions with Grant Thornton, with no members of Main Street's management present during those discussions. Grant Thornton did not identify any material audit issues, questions or discrepancies, other than those previously discussed with management, which were resolved to the satisfaction of all parties.



•

We received and reviewed the written disclosures and the letter from Grant Thornton required by the applicable requirements of the Public Company Accounting Oversight Board regarding Grant Thornton's communications with us concerning independence, and we discussed with Grant Thornton its independence from Main Street. We also considered whether the provision of non-audit services to Main Street is compatible with Grant Thornton's independence.



•

We determined that there were no former Grant Thornton employees, who previously participated in the Main Street audit, engaged in a financial reporting oversight role at Main Street.



•

We reviewed, and discussed with Main Street's management and Grant Thornton, Main Street's audited consolidated balance sheet at December 31, 2016, and consolidated statements of income, changes in net assets and cash flows for the year ended December 31, 2016.

        Based on the reviews and actions described above, we recommended to the Board of Directors that Main Street's audited financial statements be included in its annual report on Form 10-K for the year ended December 31, 2016 for filing with the Securities and Exchange Commission.

36

Table of Contents

RATIFICATION OF APPOINTMENT OF INDEPENDENT REGISTERED
PUBLIC ACCOUNTING FIRM FOR YEAR ENDING DECEMBER 31, 2017

(ITEM 2)

        Our Board of Directors has ratified the decision of the Audit Committee to appoint Grant Thornton LLP to serve as the independent registered public accounting firm to audit our financial statements for the year ending December 31, 2017. No determination has been made as to what action the Audit Committee and the Board of Directors would take if our stockholders fail to ratify the appointment. Even if the appointment is ratified, the Audit Committee retains discretion to appoint a new independent registered public accounting firm at any time if the Audit Committee concludes such a change would be in the best interests of Main Street. We expect that representatives of Grant Thornton LLP will be present at the Annual Meeting and will have an opportunity to make a statement if they desire to do so and to respond to appropriate questions.

        For the years ended December 31, 2016 and December 31, 2015, Main Street incurred the following fees for services provided by Grant Thornton, including expenses:

        Audit Fees.    Audit fees include fees for services that normally would be provided by the accountant in connection with statutory and regulatory filings or engagements and that generally only the independent accountant can provide. In addition to fees for the audit of our annual financial statements, the audit of the effectiveness of our internal control over financial reporting and the review of our quarterly financial statements in accordance with generally accepted auditing standards, this category contains fees for comfort letters, statutory audits, consents, and assistance with and review of documents filed with the SEC.

        Audit Related Fees.    Audit related fees are assurance related services that traditionally are performed by the independent accountant, such as attest services that are not required by statute or regulation.

        Tax Fees.    Tax fees include corporate and subsidiary compliance and consulting.

        All Other Fees.    Fees for other services would include fees for products and services other than the services reported above.

        It is the policy of our Audit Committee to preapprove all audit, review or attest engagements and permissible non-audit services to be performed by our independent registered public accounting firm, subject to, and in compliance with, the de minimis exception for non-audit services described in Section 10A(i)(1)(B) of the Securities Exchange Act of 1934 and the applicable rules and regulations of the SEC. Our Audit Committee did not rely on the de minimis exception for any of the fees disclosed above.

        The proxy holders will vote all proxies received for approval of this proposal unless instructed otherwise. The affirmative vote of a majority of the votes cast at the Annual Meeting, in person or by proxy, is required to ratify the appointment of Grant Thornton LLP to serve as our independent registered public accounting firm for the year ending December 31, 2017. Abstentions will not be

37

Table of Contents

included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote.

THE BOARD OF DIRECTORS RECOMMENDS YOU VOTE "FOR" THE RATIFICATION OF
GRANT THORNTON LLP AS OUR INDEPENDENT REGISTERED PUBLIC ACCOUNTING
FIRM FOR THE YEAR ENDING DECEMBER 31, 2017

ADVISORY VOTE ON EXECUTIVE COMPENSATION

(ITEM 3)

        Our Board of Directors recognizes that executive compensation is an important matter for our stockholders. As described in detail in the "Compensation Discussion and Analysis" ("CD&A") section of this proxy statement, the Compensation Committee is tasked with the implementation of our executive compensation philosophy, and the core of that philosophy has been and continues to be to pay our executives based on our and their individual performance. In particular, the Compensation Committee strives to attract, retain and motivate exceptional executives, to reward past performance and provide incentives for future performance, and to align executives' long-term interests with the interests of our stockholders. To do so, the Compensation Committee uses a combination of short- and long-term incentive compensation to reward near-term excellent performance and to encourage executives' commitment to our long-range, strategic business goals. It is always the intention of the Compensation Committee that our executive officers be compensated competitively and consistently with our strategy, sound corporate governance principles, and stockholder interests and concerns.

        As described in the CD&A, we believe our compensation program is effective, appropriate and strongly aligned with the long-term interests of our stockholders and that the total compensation package provided to our NEOs and other key employees is reasonable and not excessive. To this end, our Compensation Committee is advised from time to time by an independent compensation consultant. As you consider this proposal, we urge you to read the CD&A section of this proxy statement for additional details on executive compensation, including the more detailed information about our compensation philosophy and objectives and the past compensation of our NEOs, and to review the tabular disclosures regarding NEO compensation together with the accompanying narrative disclosures in the "Compensation of Executive Officers" section of this proxy statement.

        This non-binding advisory "Say on Pay" vote gives our stockholders the opportunity to express their views on our NEOs' compensation. This vote is not intended to address any specific item of compensation, but rather the overall compensation of our NEOs and the compensation philosophy, policies and practices described in this proxy statement. We welcome the opportunity to give our stockholders an opportunity to provide us with such a vote on executive compensation at the Annual Meeting.

        As an advisory vote, this proposal is not binding on our Board of Directors or the Compensation Committee, will not overrule any decisions made by our Board of Directors or the Compensation Committee, or require our Board of Directors or the Compensation Committee to take any specific action. Although the vote is non-binding, our Board of Directors and the Compensation Committee value the opinions of our stockholders and will carefully consider the outcome of the vote when making future compensation decisions for our NEOs and other key employees. In particular, to the extent there is any significant vote against our NEOs' compensation as disclosed in this proxy statement, we will consider our stockholders' concerns, and the Compensation Committee will evaluate whether any actions are necessary to address those concerns.

38

Table of Contents

Text of the Resolution to be Adopted

        We are asking stockholders to vote "For" the following advisory resolution:

        "RESOLVED, that the stockholders approve, on an advisory basis, the compensation philosophy, policies and procedures and the compensation of the NEOs as disclosed in the Proxy Statement for Main Street Capital Corporation's 2017 Annual Meeting of Stockholders pursuant to the compensation disclosure rules of the Securities and Exchange Commission ("SEC"), including the CD&A, the 2016 Summary Compensation Table and the other related tables and disclosures."

Required Vote

        The approval of this advisory resolution requires the affirmative vote of a majority of the votes cast at the Annual Meeting, in person or by proxy. Abstentions will not be included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote on this item.

OUR BOARD OF DIRECTORS RECOMMENDS A VOTE "FOR" THE APPROVAL OF
THIS ADVISORY RESOLUTION.

ADVISORY VOTE ON THE FREQUENCY OF THE ADVISORY VOTE
ON EXECUTIVE COMPENSATION

(ITEM 4)

        This non-binding advisory vote enables our stockholders to indicate their preference as to how frequently we should seek an advisory "Say on Pay" vote on the compensation philosophy, policies and procedures and the compensation of our NEOs, as disclosed pursuant to the SEC's compensation disclosure rules, such as the proposal in "Advisory Vote on Executive Compensation (Item 3)" included in this proxy statement. By voting on this proposal, stockholders may indicate whether they prefer that we conduct the advisory "Say on Pay" vote every three years, every two years or every year. Stockholders also may, if they wish, abstain from casting a vote on this proposal. After careful consideration of this proposal, our Board of Directors has determined that an advisory vote on executive compensation that occurs once every three years (a triennial vote) is the most appropriate alternative for our company, and therefore our Board of Directors recommends that you support a frequency period of every three years for the advisory vote on executive compensation.

        Setting a three-year period for holding this stockholder vote will enhance stockholder communication by providing a clear, simple means for our company to obtain information on investor sentiment about our executive compensation philosophy. An advisory vote once every three years will be the most effective timeframe for us to respond to stockholders' feedback by providing us with sufficient time to engage with stockholders to understand and respond to the vote results. Also, our executive compensation programs are based on our long-term business strategy, which is more appropriately assessed over a three-year timeframe. An advisory vote occurring once every three years will also permit our stockholders to observe and evaluate the impact of any changes to our executive compensation policies and practices which have occurred since the last advisory vote on executive compensation, including changes made in response to the outcome of a prior advisory vote on executive compensation. We will continue to engage with our stockholders as appropriate regarding our executive compensation program during the period between advisory votes on executive compensation.

        This vote is advisory and not binding on the Company or our Board of Directors or its Compensation Committee in any way. The Board of Directors and the Compensation Committee will take into account the outcome of the vote, however, when considering the frequency of future advisory votes on executive compensation.

39

Table of Contents

Text of the Resolution to be Adopted

        You may cast your vote on your preferred voting frequency by choosing the option of one year, two years, three years or abstain from voting when you vote in response to the resolution set forth below:

        "RESOLVED, that the option of once every one year, two years, or three years that receives the highest number of votes cast for this resolution will be deemed the preferred frequency selected by stockholders on an advisory basis with which Main Street is to hold a stockholder "Say on Pay" vote to approve the compensation philosophy, policies and procedures and the compensation of the NEOs, as disclosed pursuant to the SEC's compensation disclosure rules (which disclosure shall include the CD&A, the Summary Compensation Table, and the other related tables and disclosures)."

Required Vote

        The period receiving the greatest number of votes as set forth in this proposal will be the frequency of the advisory vote on executive compensation that has been approved by our stockholders on an advisory basis. Abstentions will not be included in determining the number of votes cast and, as a result, will not have any effect on the result of the vote on this item.

OUR BOARD OF DIRECTORS RECOMMENDS THAT STOCKHOLDERS
VOTE FOR A FREQUENCY OF "THREE YEARS"
FOR FUTURE NON-BINDING "SAY ON PAY" STOCKHOLDER
VOTES ON EXECUTIVE COMPENSATION.

40

Table of Contents

CERTAIN RELATIONSHIPS AND RELATED PARTY TRANSACTIONS

        We have procedures in place for the review, approval and monitoring of transactions involving us and certain persons related to us. As a BDC, the 1940 Act restricts us from participating in transactions with any persons affiliated with us, including our officers, directors and employees and any person controlling or under common control with us, subject to certain exceptions.

        In the ordinary course of business, we enter into transactions with portfolio companies that may be considered related party transactions. We have implemented certain policies and procedures, both written and unwritten, to ensure that we do not engage in any prohibited transactions with any persons affiliated with us. If such affiliations are found to exist, we seek Board and/or committee review and approval or exemptive relief for such transactions, as appropriate.

        In addition, our code of business conduct and ethics, which is applicable to all of our employees, officers and directors, requires that all employees, officers and directors avoid any conflict, or the appearance of a conflict, between an individual's personal interests and our interests. Our code of business conduct and ethics is available at http://mainstcapital.com under "Corporate Governance—Governance Docs" in the "Investors" section of our website.

SECTION 16(a) BENEFICIAL OWNERSHIP REPORTING COMPLIANCE

        Section 16(a) of the Securities Exchange Act of 1934 requires our directors and executive officers, and persons who own 10% or more of our voting stock, to file reports of ownership and changes in ownership of our equity securities with the SEC and the New York Stock Exchange. Directors, executive officers and 10% or more holders are required by SEC regulations to furnish us with copies of all Section 16(a) forms they file. Based solely on a review of the copies of those forms furnished to us, or written representations that no such forms were required, we believe that our directors, executive officers and 10% or more beneficial owners complied with all Section 16(a) filing requirements during the year ended December 31, 2016.

STOCKHOLDERS' PROPOSALS

        Any stockholder who wishes to have a qualified proposal considered for inclusion in our proxy statement for our 2018 Annual Meeting must send notice of the proposal to our Corporate Secretary at our principal executive office no later than November 15, 2017. If you make such a proposal, you must provide your name, address, the number of shares of common stock you hold of record or beneficially, the date or dates on which such common stock was acquired and documentary support for any claim of beneficial ownership.

        In addition, any stockholder who intends to submit a proposal for consideration at our 2018 Annual Meeting, whether or not for inclusion in our proxy materials, or who intends to submit nominees for election as directors at the meeting must notify our Corporate Secretary. Under our by-laws, such notice must (1) be received at our executive offices no earlier than September 16, 2017 or later than November 15, 2017 and (2) satisfy specified requirements.

		By Order of the Board of Directors,
		JASON B. BEAUVAIS Senior Vice President, General Counsel, Chief Compliance Officer and Secretary

Dated: March 15, 2017

41

Table of Contents

PRIVACY NOTICE

        We are committed to protecting your privacy. This privacy notice explains the privacy policies of Main Street and its affiliated companies. This notice supersedes any other privacy notice you may have received from Main Street, and its terms apply both to our current stockholders and to former stockholders as well.

        We will safeguard, according to strict standards of security and confidentiality, all information we receive about you. The only information we collect from you is your name, address, and number of shares you hold. This information is used only so that we can send you annual reports and other information about us, and send you proxy statements or other information required by law.

        We do not share this information with any non-affiliated third party except as described below.

•

The People and Companies that Make Up Main Street.  It is our policy that only our authorized employees who need to know your personal information will have access to it. Our personnel who violate our privacy policy are subject to disciplinary action.



•

Service Providers.  We may disclose your personal information to companies that provide services on our behalf, such as record keeping, processing your trades, and mailing you information. These companies are required to protect your information and use it solely for the purpose for which they received it.



•

Courts and Government Officials.  If required by law, we may disclose your personal information in accordance with a court order or at the request of government regulators. Only that information required by law, subpoena, or court order will be disclosed.

42