X0306 4 2021-09-16 0 0001643615 Montrose Environmental Group, Inc. MEG 0001501277 LeMaire Joshua W. 1 PARK PLAZA, SUITE 1000 IRVINE CA 92614 0 1 0 0 Chief Operating Officer Common Stock 2021-09-16 4 M 0 1829 6.03 A 4029 D Common Stock 2021-09-16 4 S 0 1829 60.00 D 2200 D Common Stock 2021-09-16 4 M 0 2110 24.00 A 4310 D Common Stock 2021-09-16 4 S 0 2110 60.00 D 2200 D Common Stock 2021-09-16 4 M 0 865 31.60 A 3065 D Common Stock 2021-09-16 4 S 0 865 60.00 D 2200 D Common Stock 2021-09-17 4 M 0 7095 6.03 A 9295 D Common Stock 2021-09-17 4 S 0 7095 60.08 D 2200 D Common Stock 2021-09-17 4 M 0 600 24.00 A 2800 D Common Stock 2021-09-17 4 S 0 600 60.17 D 2200 D Common Stock 2021-09-17 4 M 0 300 31.60 A 2500 D Common Stock 2021-09-17 4 S 0 300 60.18 D 2200 D Stock Option (Right to Buy) 6.03 2021-09-16 4 M 0 1829 0.00 D 2026-06-23 Common Stock 1829 186046 D Stock Option (Right to Buy) 24.00 2021-09-16 4 M 0 2110 0.00 D 2029-01-10 Common Stock 2110 18350 D Stock Option (Right to Buy) 31.60 2021-09-16 4 M 0 865 0.00 D 2030-01-01 Common Stock 865 18027 D Stock Option (Right to Buy) 6.03 2021-09-17 4 M 0 7095 0.00 D 2026-06-23 Common Stock 7095 178951 D Stock Option (Right to Buy) 24.00 2021-09-17 4 M 0 600 0.00 D 2029-01-10 Common Stock 600 17750 D Stock Option (Right to Buy) 31.60 2021-09-17 4 M 0 300 0.00 D 2030-01-01 Common Stock 300 17727 D The price reported in Column 4 is a weighted average price. These shares of common stock of the Issuer were sold in multiple transactions by the Reporting Person at prices ranging from $60.00 to $60.40, inclusive. The Reporting Person undertakes to provide upon request by the Commission staff, the Issuer, or a security holder of the Issuer full information regarding the number of shares of common stock sold at each separate price within the range. The price reported in Column 4 is a weighted average price. These shares of common stock of the Issuer were sold in multiple transactions by the Reporting Person at prices ranging from $60.00 to $60.36, inclusive. The Reporting Person undertakes to provide upon request by the Commission staff, the Issuer, or a security holder of the Issuer full information regarding the number of shares of common stock sold at each separate price within the range. The price reported in Column 4 is a weighted average price. These shares of common stock of the Issuer were sold in multiple transactions by the Reporting Person at prices ranging from $60.00 to $60.35, inclusive. The Reporting Person undertakes to provide upon request by the Commission staff, the Issuer, or a security holder of the Issuer full information regarding the number of shares of common stock sold at each separate price within the range. 50% of the shares subject to the option vested on July 13, 2017 and the remaining 50% vested on July 13, 2019. 33.33% of the shares subject to the option vested on January 10, 2020, 33.33% vested on January 10, 2021 and the remaining balance will vest on January 10, 2022, subject to the Reporting Person's continued service to the Issuer. 33.33% of the shares subject to the option vested on January 1, 2021, 33.33% will vest on January 1, 2022 and the remaining balance will vest on January 1, 2023, subject to the Reporting Person's continued service to the Issuer. The transactions reported in this Form 4 were effected pursuant to a Rule 10b5-1 Trading Plan adopted by the Reporting Person. /s/ Nasym Afsari, Attorney In Fact 2021-09-17