 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 was recalled by the Islanders and made his NHL debut, going scoreless, on April 10, 2016.Johnston started the 2017–18 season with the Bridgeport Sound Tigers, but was recalled on January 19, 2018, to replace the injured Casey Cizikas.