Theodore began his minor hockey career with the Aldergrove Minor Hockey Association, during which he established himself as the most outstanding player in the team's franchise history. His successful minor career earned him a selection in the 2010 WHL Bantam Draft, by the Seattle Thunderbirds. During his tenure with the team, Theodore served as captain and competed for Team Canada at various international tournaments. He won a gold medal at the 2013 IIHF World U18 Championships and 2015 World Junior Ice Hockey Championships and a silver medal at the 2019 IIHF World Championship.