v3.10.0.1
Document and Entity Information - shares
6 Months Ended
Sep. 30, 2018
Oct. 31, 2018
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Sep. 30, 2018  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
Trading Symbol MIME  
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   60,063,419