The Islanders signed Johnston as a free agent during his final season of junior hockey in 2014-15. Johnston played in the NHL, AHL and ECHL during his first professional season in 2015-16, making his NHL debut on April 10 against the Philadelphia Flyers.
In 141 career AHL games with Bridgeport, Johnston has 30 points (12 goals, 18 assists) and collected 327 penalty minutes. He has appeared in 13 ECHL regular-season games and five playoff games with the Missouri Mavericks during the 2015-16 season.
Johnston played four years of junior hockey in the Quebec Major Junior Hockey League with the Moncton Wildcats, Victoriaville Tigers and Charlottetown Islanders. He set junior career highs in goals (18) and points (32) in 44 games during his fourth season, serving as an alternate captain with Charlottetown.
