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.[3] He returned to the AHL for the 2016–17 season and started the 2017–18 season with the Sound Tigers.[4] Johnston was recalled on January 19, 2018, to replace the injured Casey Cizikas.[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.[6] He recorded his first NHL goal on January 25, against the Vegas Golden Knights to help the Islanders win 2–1.[7] Johnston finished the season appearing in 24 games with the Islanders recording three goals and six points.[8]
As a restricted free agent, Johnston agreed to a four-year contract extension with the Islanders on July 9, 2018.[8] On October 26, 2021, Johnston signed a four-year contract extension with the Islanders.[9]