X0306 4 2019-02-12 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 2019-02-12 4 M 0 5000 19.28 A 5000 D Ordinary Shares 2019-02-12 4 S 0 5000 47.25 D 0 D Restricted Share Units 5000 D Share Option (Right to Buy) 19.28 2019-02-12 4 M 0 5000 0.00 D 2016-10-03 Ordinary Shares 5000 143000 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on September 14, 2018. 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 2019-02-13