Johnston played junior ice hockey starting in the MHL with the Summerside Western Capitals before graduating to play three seasons in the Quebec Major Junior Hockey League (QMJHL) with the Moncton Wildcats, Victoriaville Tigres and Charlottetown Islanders.

As a free agent, he attended the NHL's New York Islanders training camp in 2014, but had his jaw broken in a fight.

[1][2] At the end of the 2015–16 season, his first professional year, Johnston split his time between the Islanders' American Hockey League (AHL) affiliate, the Bridgeport Sound Tigers and the Missouri Mavericks of the ECHL.

[5] He recorded his first NHL point on January 21, assisting on a goal by Brock Nelson in the third period of a 7–3 win against the Chicago Blackhawks.

[8] As a restricted free agent, Johnston agreed to a four-year contract extension with the Islanders on July 9, 2018.