 After going undrafted in the 2016 NFL Draft, Johnstone signed as a free agent with the San Diego Chargers of the National Football League on April 30, 2016.He spent the season on the injured reserve and was released in April 2017.After obtaining dual Canadian-U.S. citizenship, Johnstone was drafted by the Alouettes on July 2, 2018 in the 2018 Supplemental CFL Draft, meaning that the Alouettes forfeited a first-round selection in the 2019 CFL Draft to obtain his playing rights.