In his rookie season in 2019–20, Johnston scored his first OHL goal on September 21, 2019 against the Peterborough Petes in a 9–6 victory.
Johnston went on to score 12 goals and 18 assists for 30 points in 53 games in his rookie year before the season was cancelled due to the COVID-19 pandemic.
Johnston made his NHL debut in the Stars' first game of the season on October 13, 2022, against the Nashville Predators.
On January 28, 2025, Johnston became the youngest player in franchise history to score a natural hat-trick in a game against Vegas.
[6] During his rookie season with the Dallas Stars, Johnston lived in the house of teammate Joe Pavelski.