X0306 4 2019-04-11 0 0001644675 Mimecast Ltd MIME 0001734798 Jennings Edward Vincent C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Chief Operating Officer Ordinary Shares 2019-04-11 4 M 0 11000 9.78 A 12251 D Ordinary Shares 2019-04-11 4 S 0 11000 47.9513 D 1251 D Restricted Share Units 6750 D Restricted Share Units 36000 D Share Option (Right to Buy) 9.78 2019-04-11 4 M 0 11000 0.00 D 2025-07-31 Ordinary Shares 11000 185088 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on December 3, 2018. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $47.82 - $48.18, 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 vested 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. These restricted share units were granted on April 1, 2019. Twenty-five percent (25%) of the units vest 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 such vesting date. Represents shares underlying unexercised share options that were granted on August 1, 2015. The terms of the original grant of 708,088 options provide that twenty-five percent (25%) of the shares vested on August 1, 2016 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 2019-04-15