X0306 4 2021-02-01 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 2021-02-01 4 M 0 4298 A 8663 D Restricted Share Units 2021-02-01 4 F 0 1952 44.45 D 16798 D Restricted Share Units 2021-02-01 4 M 0 4298 D 12500 D Ordinary Shares 2021-02-02 4 S 0 4243 45.145 D 4420 D Ordinary Shares 2021-02-02 4 S 0 55 45.78 D 4365 D Restricted Share Units 12732 D The number of Ordinary Shares reflects the settlement of restricted share units upon vesting, net of Ordinary Shares that were retained (but not issued) by the Issuer in satisfaction of tax withholding obligations associated with the vesting of such units. Each restricted share unit represents a contingent right to receive one share of the Issuer's Ordinary Shares. The restricted share units reported as disposed of herein reflect the number of Ordinary Shares underlying the restricted share units that were retained (but not issued) by the Issuer in satisfaction of tax withholding obligations associated with the vesting of such units. Price reflects the closing price of the Issuer's Ordinary Shares on the Nasdaq Global Select Market on February 1, 2021. 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. The restricted share units reported as disposed of herein were settled for the Issuer's Ordinary Shares upon the vesting of such units (as reported in Row 1 above). 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 $44.735 - $45.605, 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. 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. 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 2021-02-03