Coming to the Bucks from Manitoba, Moxham was honored as 2015 Manitoba Player of the Year while playing at Vauxhall High School. He went on to pitch for Team Canada at the 2015 World 18U World Championships, hurling 10.2 innings in tournament appearances against Japan and South Korea and allowing only 3 runs and 2 walks with 7 strikeouts. He started his college career in 2016 at Midland College, going 8-2 in 14 games and 13 starts. He struck out 52 across 69.1 innings to help lead the Chaparrals to a Conference Title. Moxham then transferred to South Dakota State University, redshirting in 2017. In his first season with the Jackrabbits in 2018, he started 16 games and threw at a 5.55 ERA with a 1-6 record. In 73.0 frames he whiffed 68 and held opponents to a .264 batting average against.