UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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☐ Preliminary proxy statement

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☐ Definitive proxy statement

☒ Definitive additional materials

☐ Soliciting material pursuant to § 240.14a-12

Montrose Environmental Group, Inc.

(Name of registrant as specified in its charter)

N/A

(Name of person(s) filing proxy statement, if other than the registrant)

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MONTROSE ENVIRONMENTAL VOTE Online Go to www.envisionreports.com/MEG or scan the QR code – login details are located in the shaded bar below. Votes submitted electronically must be received by 9:30am PT, on May 13, 2021. Annual Stockholder Meeting Notice Important Notice Regarding the Availability of Proxy Materials for the MONTROSE ENVIRONMENTAL GROUP, INC. Shareholder Meeting to be Held on Thursday May 13, 2021 at 9:00 am Pacific Time Under Securities and Exchange Commission rules, you are receiving this notice that the proxy materials for the annual shareholders’ meeting are available on the Internet. Follow the instructions below to view the materials and vote online or request a copy. The items to be voted on and location of the annual meeting are on the reverse side. Your vote is important! This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. We encourage you to access and review all of the important information contained in the proxy materials before voting. The proxy statement and annual report to shareholders are available at: www.envisionreports.com/MEG Easy Online Access – View your proxy materials and vote. Step 1: Go to www.envisionreports.com/MEG. Step 2: Click on Cast Your Vote or Request Materials. Step 3: Follow the instructions on the screen to log in. Step 4: Make your selections as instructed on each screen for your delivery preferences. Step 5: Vote your shares. When you go online, you can also help the environment by consenting to receive electronic delivery of future materials. Obtaining a Copy of the Proxy Materials – If you want to receive a copy of the proxy materials, you must request one. There is no charge to you for requesting a copy. Please make your request as instructed on the reverse side on or before May 3, 2021 to facilitate timely delivery. 2NOT 03FKMA

Annual Stockholder Meeting Notice MONTROSE ENVIRONMENTAL GROUP, INC. Annual Meeting of Stockholders will be held virtually on Thursday, May 13, 2021 at 9:00 a.m. Pacific Time via the internet at https://register.proxypush.com/MEG. Please check the proxy materials for instructions on how to access and vote at the Annual Meeting. Proposals to be voted on at the meeting are listed below along with the Board of Directors’ recommendations. The Board of Directors recommend a vote FOR each of the nominees listed in Proposal 1 and FOR Proposal 2: 1. Election of Directors: 01 - J. Miguel Fernandez de Castro 02 - Vijay Manthripragada 03 - Robin L. Newmark 2. To ratify the appointment of Deloitte & Touche LLP as independent registered public accounting firm for the Company for the fiscal year ending December 31, 2021 PLEASE NOTE – YOU CANNOT VOTE BY RETURNING THIS NOTICE. To vote your shares you must go online or request a paper copy of the proxy materials to receive a proxy card. Here’s how to order a copy of the proxy materials and select delivery preferences: Current and future delivery requests can be submitted using the options below. If you request an email copy, you will receive an email with a link to the current meeting materials. PLEASE NOTE: You must use the number in the shaded bar on the reverse side when requesting a copy of the proxy materials. – Internet - Go to www.envisionreports.com/MEG. Click Cast Your Vote or Request Materials. – Phone - Call us free of charge at 1-866-641-4276. – Email - Send an email to investorvote@computershare.com with “Proxy Materials Montrose (MEG)” in the subject line. Include your full name and address, plus the number located in the shaded bar on the reverse side, and state that you want a paper copy of the meeting materials. To facilitate timely delivery, all requests for a paper copy of proxy materials must be received by May 3, 2021.