Johnston started his collegiate career at Frank Phillips Junior College. Throughout two seasons, he delivered 102 innings, as he struck out 88 batters, while walking 23. After Junior College, Johnston played for Division II East Central University from 2020 to 2021, holding a 5.56 ERA across 84.2 innings pitched, while striking out 72 batters. Johnston rounded out his collegiate career as he spent the 2022 season at Division II Arkansas-Monticello, posting a 4.19 ERA in 86 innings, while striking out 71 hitters. Upon graduation, Johnston played professionally during the summer of 2022 for the Joliet Slammers of the Frontier League as he pitched 51.2 innings and held a 2.79 ERA while posting a 5-3 record. After the conclusion of his rookie year, Johnston was traded to the Washington WildThings, where he spent part of the 2023 season. He pitched in six games with a 6.35 ERA. Johnston moved to the Atlantic League for the remainder of 2023, starting with the Lancaster Barnstormers, as he pitched in five games while holding a 6.58 ERA. Johnston finished his 2023 campaign with the Southern Maryland Blue Crabs and found his stride as he started all 11 of his appearances while maintaining a 3.66 ERA. Johnston returned to Southern Maryland for the 2024 season as he delivered 81.1 innings in 15 appearances, striking out 69 batters while posting a 5.98 ERA.