Defined Contribution Plan	3 Months Ended
Mar. 31, 2021
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	21. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of March 31, 2021 and December 31, 2020, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. Prior to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Beginning on May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions were reinstated beginning on April 23, 2021. Employer contributions under the 401(k) Savings Plan for the three months ended March 31, 2021 and March 31, 2020 were zero and $0.8 million, respectively, and are included within selling, general, and administrative expenses on the unaudited condensed consolidated statements of operations.

Defined Contribution Plan

3 Months Ended

Mar. 31, 2021

Compensation And Retirement Disclosure [Abstract]

21. DEFINED CONTRIBUTION PLAN

On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan (the “401(k) Savings Plan”). As of March 31, 2021 and December 31, 2020, plan participants may defer up to 85.0% of their eligible wages for the year, up to the Internal Revenue Service dollar limit and catch up contribution allowed by law. Prior to May 22, 2020, the Company provided employer matching contributions equal to 100.0% of the first 3.0% of the participant’s compensation and 50.0% of the participant’s elective deferrals that exceed 3.0% but do not exceed 4.0% of the participant’s compensation. Beginning on May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions were reinstated beginning on April 23, 2021. Employer contributions under the 401(k) Savings Plan for the three months ended March 31, 2021 and March 31, 2020 were zero and $0.8 million, respectively, and are included within selling, general, and administrative expenses on the unaudited condensed consolidated statements of operations.