| Document and Entity Information | Mar. 11, 2022 |
| Cover [Abstract] | |
| Entity Registrant Name | Mimecast Ltd |
| Entity Tax Identification Number | 00-0000000 |
| Amendment Flag | false |
| Entity Central Index Key | 0001644675 |
| Document Type | 8-K |
| Document Period End Date | Mar. 11, 2022 |
| Entity Incorporation State Country Code | Y9 |
| Entity File Number | 001-37637 |
| Entity Address, Address Line One | 1 Finsbury Avenue |
| Entity Address, City or Town | London |
| Entity Address, Postal Zip Code | EC2M 2PF |
| Entity Address, Country | GB |
| City Area Code | (781) |
| Local Phone Number | 996-5340 |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Security 12b Title | Ordinary Shares, $0.012 par value per share |
| Trading Symbol | MIME |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
