Defined Contribution Plan	12 Months Ended
Dec. 31, 2020
Compensation And Retirement Disclosure [Abstract]
Defined Contribution Plan	22. DEFINED CONTRIBUTION PLAN On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of December 31, 2020, 2019, and 2018, 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. Up 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. Subsequent to May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions for years ended December 31, 2020, 2019, and 2018 were $1.2 million, $2.6 million and $2.1 million, respectively, and are included within selling, general, and administrative expenses on the consolidated statements of operations.

Defined Contribution Plan

12 Months Ended

Dec. 31, 2020

Compensation And Retirement Disclosure [Abstract]

22. DEFINED CONTRIBUTION PLAN

On January 1, 2014, the Company established the Montrose Environmental Group 401(k) Savings Plan. As of December 31, 2020, 2019, and 2018, 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. Up 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. Subsequent to May 22, 2020, the Company temporarily ceased making employer contributions. Employer contributions for years ended December 31, 2020, 2019, and 2018 were $1.2 million, $2.6 million and $2.1 million, respectively, and are included within selling, general, and administrative expenses on the consolidated statements of operations.