| Cover | Mar. 07, 2022 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 7, 2022 |
| Current Fiscal Year End Date | --12-31 |
| Entity File Number | 001-36067 |
| Entity Registrant Name | Mandiant, Inc. |
| Entity Central Index Key | 0001370880 |
| Entity Tax Identification Number | 20-1548921 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 11951 Freedom Drive, 6th Floor |
| Entity Address, City or Town | Reston |
| Entity Address, State or Province | VA |
| Entity Address, Postal Zip Code | 20190 |
| City Area Code | 703 |
| Local Phone Number | 935-1700 |
| Written Communications | false |
| Soliciting Material | true |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.0001 per share |
| Trading Symbol | MNDT |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
