Document and Entity Information - shares
6 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2019  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus Q2  
Trading Symbol MIME  
Title of 12(b) Security Ordinary Shares, nominal value $0.012 per share  
Security Exchange Name NASDAQ  
Entity Registrant Name Mimecast Ltd  
Entity Central Index Key 0001644675  
Current Fiscal Year End Date --03-31  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   61,991,355
Entity Shell Company false  
Document Quarterly Report true  
Document Transition Report false  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity File Number 001-37637  
Entity Incorporation, State or Country Code Y9  
Entity Tax Identification Number 00-0000000  
Entity Address, Address Line One 1 Finsbury Avenue  
Entity Address, City or Town London  
Entity Address, Country GB  
Entity Address, Postal Zip Code EC2M 2PF  
City Area Code 781  
Local Phone Number 996-5340