X0306 4 2018-06-01 0 0001644675 Mimecast Ltd MIME 0001010502 NAULT ROBERT P C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Sr. Vice President and GC Ordinary Shares 2018-06-01 4 M 0 12000 19.28 A 12000 D Ordinary Shares 2018-06-01 4 S 0 12000 44.2881 D 0 D Restricted Share Units 5000 D Share Option (Right to Buy 19.28 2018-06-01 4 M 0 12000 0.00 D 2026-10-01 Ordinary Shares 12000 148000 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on March 9, 2018. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $44.00 - $44.80, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. Each restricted share unit represents a contingent right to receive one share of the Issuer's ordinary shares. These restricted share units were granted on April 2, 2018. Twenty-five percent (25%) of the units vest on April 2, 2019 and the remainder of the units vest on an annual basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on such vesting date. This option was granted on October 3, 2016. Twenty-five percent (25%) of the shares vested on October 3, 2017 and the remainder of the shares vest on a quarterly basis over the next three (3) years, provided that the Reporting Person remains an employee of the Issuer on such vesting date. /s/ Robert P. Nault 2018-06-05