X0306 4 2021-06-10 0 0001644675 Mimecast Ltd MIME 0001762176 Anderson Karen M. C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Chief Human Resources Officer Ordinary Shares 2021-06-10 4 M 0 27654 40.00 A 32019 D Ordinary Shares 2021-06-10 4 S 0 27654 52.1946 D 4365 D Ordinary Shares 2021-06-11 4 M 0 7735 40.00 A 12100 D Ordinary Shares 2021-06-11 4 S 0 7735 52.0153 D 4365 D Restricted Share Units 12500 D Restricted Share Units 9549 D Restricted Share Units 12239 D Share Option (Right to Buy) 40.00 2021-06-10 4 M 0 27654 0.00 D 2029-02-01 Ordinary Shares 27654 93032 D Share Option (Right to Buy) 40.00 2021-06-11 4 M 0 7735 0.00 D 2029-02-01 Ordinary Shares 7735 85297 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on November 16, 2020. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $51.99 - $52.49, 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. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $51.99 - $52.05, 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 February 1, 2019. Twenty-five percent (25%) of the units vested on February 1, 2020, twenty-five percent (25%) of the units vested on February 1, 2021, 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 such vesting date. These restricted share units were granted on April 1, 2020. Twenty-five percent (25%) of the units vested 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. These restricted share units were granted on April 1, 2021. Twenty-five percent (25%) of the units vest on April 1, 2022 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 February 1, 2019. Twenty-five percent (25%) of the shares vested on February 1, 2020, 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 January 3, 2019). /s/ Robert P. Nault, Attorney-in-Fact 2021-06-14