Document and Entity Information - shares
3 Months Ended
Jun. 30, 2019
Jul. 31, 2019
Document And Entity Information [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2019  
Document Fiscal Year Focus 2020  
Document Fiscal Period Focus Q1  
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   61,759,379
Entity Shell Company false  
Entity Current Reporting Status Yes  
Entity File Number 001-37637  
Entity Address, Address Line One CityPoint, One Ropemaker Street  
Entity Address, City or Town Moorgate  
Entity Address, State or Province London  
Entity Address, Country United Kingdom  
Entity Address, Postal Zip Code EC2Y 9AW  
City Area Code 781  
Local Phone Number 996-5340