Johnston posted a 9-7 record for the Bird Dawgs in 2025, leading the team with 103.1 innings pitched. He made 18 appearances, all starts, while recording a 4.70 ERA with 64 strikeouts and 31 walks, including earning the Opening Day start.

He began his collegiate career at Frank Phillips Junior College, where he logged 102 innings over two seasons, striking out 88 batters while issuing just 23 walks. Johnston then moved to Division II East Central University, where he pitched from 2020 to 2021, posting a 5.56 ERA across 84.2 innings. He concluded his collegiate career at Division II Arkansas-Monticello in 2022, recording a 4.19 ERA over 86 innings pitched.

Following graduation, Johnston began his professional career with the Joliet Slammers of the Frontier League in 2022, where he went 5-3 with a 2.79 ERA over 51.2 innings pitched. He was later traded to the Washington Wild Things, appearing in six games during the 2023 season.

Johnston also spent time in the Atlantic League in 2023, beginning with the Lancaster Barnstormers before finishing the season with the Southern Maryland Blue Crabs. With Southern Maryland, he found his stride, starting all 11 of his appearances and posting a 3.66 ERA. He returned to the Blue Crabs in 2024, logging 81.1 innings over 15 appearances while striking out 69 batters.