[1] Johnstone was born on June 19, 1996, in Cranford, New Jersey to parents Thomas and Robin.
[4][5] 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.
On September 2, 2021, the Newfoundland Growlers of the ECHL signed Johnstone to a standard, one-year contract.
On June 19, 2022, the Marlies signed Johnstone to a standard, one-year contract.
On July 2, 2023, the Pittsburgh Penguins of the National Hockey League (NHL) signed Johnstone to a two-year, entry-level contract.