X0306 4 2020-10-15 0 0001644675 Mimecast Ltd MIME 0001267550 WALSH JOHN J JR C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Sr. Vice Pres., Engineering Ordinary Shares 2020-10-15 4 M 0 2000 24.45 A 5697 D Ordinary Shares 2020-10-15 4 S 0 2000 46.12 D 3697 D Restricted Share Units 1650 D Restricted Share Units 850 D Restricted Share Units 8250 D Restricted Share Units 29958 D Share Option (Right to Buy) 24.45 2020-10-15 4 M 0 2000 0.00 D 2027-05-01 Ordinary Shares 2000 150500 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 12, 2020. 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 vested on April 2, 2019, twenty-five percent (25%) of the units vested on April 2, 2020, and the remainder of the units vest on an annual basis over the next two (2) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on June 1, 2018. Twenty-five percent (25%) of the units vested on June 1, 2019, twenty-five percent (25%) of the units vested on June 1, 2020, and the remainder of the units vest on an annual basis over the next two (2) years, provided that the Reporting Person remains an employee of the Issuer on each such vesting date. These restricted share units were granted on April 1, 2019. Twenty-five percent (25%) of the units vested on April 1, 2020 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 each such vesting date. These restricted share units were granted on April 1, 2020. Twenty-five percent (25%) of the units vest on April 1, 2021 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 each such vesting date. This option was granted on May 1, 2017. Twenty-five percent (25%) of the shares vested on May 1, 2018 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. Exhibit 24 Power of Attorney (incorporated by reference to Power of Attorney filed as Exhibit 24 to Form 3 filed by the Reporting Person on April 2, 2018). /s/ Robert P. Nault, Attorney-in-Fact 2020-10-15