MAINSTAY MACKAY DEFINEDTERM MUNICIPAL

OPPORTUNITIES FUND

51 MADISON AVENUE

NEW YORK, NEW YORK 10010

ANNUAL MEETING OF SHAREHOLDERS

TO BE HELD ON SEPTEMBER 28, 2022

To Our Shareholders:

I am writing to inform you of the upcoming Annual Meeting of Shareholders (with any postponements or adjournments, "Meeting") of the MainStay MacKay DefinedTerm Municipal Opportunities Fund ("Fund"), a closed-end management investment company organized as a Delaware statutory trust. The Meeting will be held telephonically on September 28,

2022 beginning at 9:00 a.m., Eastern time. The purpose of the Meeting is:

  1. To elect two Trustees to serve as Class I Trustees of the Fund for three-year terms or until their successors are duly elected and qualify; and
  2. To transact such other business as may properly come before the Meeting and any adjournments or postponements thereof.

The Board of Trustees ("Board") of the Fund has reviewed the qualifications and backgrounds of the Class I Trustee nominees and believes that they are experienced in overseeing an investment company, are familiar with the Fund and its manager and subadvisor and that their election is in the Fund's best interests. Therefore, the Board recommends that you vote "FOR"

the Class I Trustee nominees.

Due to the continued public health impact of the coronavirus pandemic (COVID-19), to mitigate potential risks to the health and safety of our shareholders and advisory and administrative personnel and to support the health and well-being of our shareholders, including social distancing measures, you will not be able to attend the Meeting in person. All shareholders are requested to vote over the Internet, by telephone or by completing, dating and signing the enclosed proxy card and returning it promptly. You may also vote at the telephonic Meeting if you choose to participate telephonically. To participate in the Meeting, please email shareholdermeetings@computershare.com at least 3 days prior to the Meeting date. You will need to provide your name, address and control number in order to gain access to the call-in number and passcode. You may vote during the Meeting by following the instructions provided during the Meeting.

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To avoid unnecessary expenses, I ask for your cooperation in responding promptly. I encourage you to carefully read the full text of the enclosed Proxy Statement, and vote according to the manner specified, either by mail, on the Internet, by phone or at the Meeting.

Your vote is very important to us regardless of the number of shares of the Fund you own. Whether or not you plan to attend the telephonic Meeting, please read the Proxy Statement and cast your vote promptly. It is important that your vote be received by no later than the time of the Meeting on September 28, 2022. A proxy card accompanies the Proxy Statement. There are several ways to vote your shares. Please refer to the proxy card for more information on how to vote. If you have any questions before you vote, please call toll-free866-963-5746. We will get you the answers that you need.

We appreciate your participation and prompt response in this matter and thank you for your continued support.

Sincerely,

Kirk C. Lehneis

President

Enclosure

MSMHI84a-07/22

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MAINSTAY MACKAY DEFINEDTERM MUNICIPAL

OPPORTUNITIES FUND

51 MADISON AVENUE

NEW YORK, NEW YORK 10010

NOTICE OF ANNUAL MEETING OF SHAREHOLDERS

TO BE HELD ON SEPTEMBER 28, 2022

The Proxy Statement is also available at:

www.proxy-direct.com/man-32816

To Our Shareholders:

Notice is hereby given that the Annual Meeting of Shareholders (with any postponements and adjournments, "Meeting") of MainStay MacKay DefinedTerm Municipal Opportunities Fund ("Fund"), a closed-endmanagement investment company organized as a Delaware statutory trust, will be held telephonically on September 28, 2022, beginning at 9:00 a.m., Eastern time for the following purposes:

  1. To elect two Trustees to serve as Class I Trustees of the Fund for three-year terms or until their successors are duly elected and qualify; and
  2. To transact such other business as may properly come before the Meeting and any adjournments or postponements thereof.

Your attention is directed to the accompanying Proxy Statement for

further information regarding the Meeting. The Board of Trustees ("Board") of the Fund has fixed July 6, 2022 as the record date for the determination of shareholders entitled to notice of, and to vote at, the Meeting or any adjournments or postponements thereof, and only holders of shares on that date are entitled to notice of, and to vote at, the Meeting and any adjournments or postponements thereof.

You are cordially invited to attend the Meeting by participating telephonically. If you attend the Meeting, you may vote your shares telephonically. Even if you do not attend the Meeting, you may authorize your proxy by simply: (i) completing, signing, and returning the enclosed proxy card by mail in the postage-paid envelope provided; or (ii) following the instructions on the voting instruction card for authorizing your proxy. Please refer to the proxy card for more information on how you may vote. You may revoke your proxy at any time prior to the date the proxy is to be exercised in the manner described in the Proxy Statement.

Due to the continued public health impact of the coronavirus pandemic (COVID-19), to mitigate potential risks to the health and safety of our

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shareholders and advisory and administrative personnel and to support the health and well-being of our shareholders, including social distancing measures, you will not be able to attend the Meeting in person. All shareholders are requested to vote over the Internet, by telephone or by completing, dating and signing the enclosed proxy card and returning it promptly. You may also vote at the telephonic Meeting if you choose to participate telephonically. To participate in the Meeting, please email shareholdermeetings@computershare.com at least 3 days prior to the Meeting date. You will need to provide your name, address and control number in order to gain access to the call-in number and passcode. You may vote during the Meeting by following the instructions provided during the Meeting.

Your vote is very important to us. Whether or not you plan to attend the telephonic Meeting, please vote using the enclosed proxy. If you have any questions before you vote, please call toll-free866-963-5746.

By Order of the Board of Trustees,

J. Kevin Gao

Chief Legal Officer and Secretary

July 18, 2022

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IMPORTANT NOTICE:

PLEASE VOTE USING THE ENCLOSED PROXY AS SOON AS

POSSIBLE. YOUR VOTE IS VERY IMPORTANT TO US NO MATTER

HOW MANY SHARES YOU OWN. YOU CAN HELP AVOID THE ADDITIONAL EXPENSE OF FURTHER SOLICITATIONS BY PROMPTLY VOTING THE ENCLOSED PROXY.

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Mainstay MacKay DefinedTerm Municipal Opportunities Fund published this content on 28 September 2022 and is solely responsible for the information contained therein. Distributed by Public, unedited and unaltered, on 27 September 2022 00:10:06 UTC.