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. He went undrafted by teams in the
National Hockey League (NHL). As a free agent, he attended the NHL's
New York Islanders training camp in 2014, but had his jaw broken in a fight. Due to the injury, he missed two months of his final season with Charlottetown. On March 31, 2015, Johnston was signed to a three-year, entry-level contract with the New York Islanders. 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. He was recalled by the Islanders and made his NHL debut on April 10, 2016. He returned to the AHL for the
2016–17 season and started the
2017–18 season with the Sound Tigers. Johnston was recalled on January 19, 2018, to replace the injured
Casey Cizikas. 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. He recorded his first NHL goal on January 25, against the
Vegas Golden Knights to help the Islanders win 2–1. Johnston finished the season appearing in 24 games with the Islanders recording three goals and six points. As a
restricted free agent, Johnston agreed to a four-year contract extension with the Islanders on July 9, 2018. Johnston made the Islanders
2023–24 season opening roster on October 9, 2023, but was
waived and claimed by the
Anaheim Ducks on October 10. He made his Ducks debut on opening night, October 14, against the
Vegas Golden Knights. ==Personal life==