X0306 4 2018-07-16 0 0001644675 Mimecast Ltd MIME 0001734756 Campbell Peter Andrew James C/O MIMECAST NORTH AMERICA, INC. 191 SPRING STREET LEXINGTON MA 02421 0 1 0 0 Chief Financial Officer Ordinary Shares 2018-07-16 4 M 0 30000 9.21 A 250000 D Ordinary Shares 2018-07-16 4 S 0 30000 42.0938 D 220000 D Ordinary Shares 4571 I By Spouse Restricted Share Units 8000 D Share Option (Right to Buy) 9.21 2018-07-16 4 M 0 30000 0.00 A 2026-02-01 Ordinary Shares 30000 105000 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 13, 2017. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $41.75 - $42.64, 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 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. Represents shares underlying unexercised share options that were granted on February 1, 2016. The terms of the original grant of 240,000 options provide that twenty-five percent (25%) of the shares vested on February 1, 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 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 2018-07-18