A long-time standout “funky” reliever for the University of Maine, Johnson played for former Maine standout Aaron Izaryk at Bridgton Academy. He was named a captain as a senior at Taunton High School, and finished his high school career 16-5 with a 1.70 ERA with 137 strikeouts in 133 innings. Johnson was named a 2013 Hockomock League all-star, a two-time Taunton Gazette all-scholastic, a 2012 Old Colony League All-Star, and also played hockey and golf. Beginning his career at Maine, he pitched 18 innings as a freshman in 2015 with a .676 opponent OPS and a 4.50 ERA plus two saves out of the bullpen. As a sophomore, Johnson went 4-4 over 21 appearances and 37.1 innings, posting another stellar opponent OPS at .680 with a 24% strikeout rate and a 4.10 ERA. Over 38.0 frames as a junior, he again had a 4-4 record with a 3.52 FIP and a save. As a senior in 2018, Johnson had a 4.33 ERA with a pair of saves in 35.1 innings.