After competing in four seasons at
Sacred Heart University, Johnstone signed a contract with the
South Carolina Stingrays of the
ECHL on March 25, 2021. Johnstone would finish out
the season, playing in five game with the Stingrays. On September 2, 2021, the
Newfoundland Growlers of the ECHL signed Johnstone to a standard, one-year contract. Johnstone also appeared in four games with the Growlers' AHL affiliate, the
Toronto Marlies, of the
American Hockey League (AHL). On June 19, 2022, the Marlies signed Johnstone to a standard, one-year contract. Johnston spent the entirety of the season with the Marlies, playing 69 of 72 possible regular season games, whilst appearing in all seven of the Marlies' postseason games. On July 2, 2023, the
Pittsburgh Penguins of the
National Hockey League (NHL) signed Johnstone to a two-year, entry-level contract. On December 8, 2023, Johnstone made his NHL debut with the Penguins. After two seasons within the Penguins organization, Johnstone left as a free agent and returned to the Toronto Marlies on a two-year AHL contract on July 7, 2025. ==Career statistics==