Johnston didn't have much time on ice to make an impact in 2020-21, but the Dallas Stars saw enough to make him the No. 23 pick of the 2021 NHL Draft and more than enough to give him a five-year, $42 million contract he signed March 8, 2025. At 22 years, 183 days, he became the third-youngest player in Stars/Minnesota North Stars history to score 100 goals during a 7-0 win at the Montreal Canadiens on Nov. 13. Only Brian Bellows (20 years, 179 days) and Mike Modano (22 years, 173 days) reached the mark at a younger age.

On Jan. 28 of that year, Johnston became the youngest player in Dallas/Minnesota North Stars history to complete a natural hat trick when he scored 20 seconds into overtime for a 4-3 victory against the Vegas Golden Knights at T-Mobile Arena. His goal at 12:48 of the third period in Game 7 of the 2023 Western Conference Second Round was the series-clincher for the Stars in a 2-1 win against the Seattle Kraken. It made him the youngest player in NHL history (20 years, one day) to score a series-clinching goal in Game 7, besting Jaromir Jagr (20 years, 76 days) for the Pittsburgh Penguins in the 1992 Patrick Division Semifinals.

With the Ontario Hockey League canceling its 2020-21 season due to concerns surrounding the coronavirus, Johnston's only hockey came with Canada at the 2021 IIHF World Under-18 Championship. Playing a checking role, he scored four points (two goals, two assists) in seven games to help Canada win the tournament.

Johnston had a strong rookie season in the OHL in 2019-20, scoring 30 points (12 goals, 18 assists) in 53 games for Windsor. He led all OHL skaters with 124 points (46 goals, 78 assists) in 68 games of the 2021-22 season.

Johnston (19 years, 151 days) became the second-youngest player in Dallas Stars/Minnesota North Stars history to score in his NHL debut, a 4-1 win against the Nashville Predators on Oct. 13, 2022. Dave Archibald (18 years, 179 days) scored twice for Minnesota on Oct. 10, 1987. He ended his rookie season tied with Seattle Kraken forward Matty Beniers for the lead among NHL rookies with 24 goals, fourth with 41 points, second with 160 shots on goal and third with 34 even-strength points (21 goals) in 82 regular-season games. He was the fourth teenager to play for general manager Jim Nill since he arrived in Dallas in 2013, joining Miro Heiskanen, Denis Gurianov and Valeri Nichushkin.