[11] He was assigned to the Ducks American Hockey League (AHL) affiliate, the Norfolk Admirals, on April 10, 2014[12] but recorded no points in four games.

[14] In his final season of junior hockey, Theodore served an assistant captain[2] and was awarded the Bill Hunter Memorial Trophy as WHL defenceman of the year.

[18] He would later record his first career NHL goal in a game against the Ottawa Senators on January 13, 2016,[19] and help the Ducks qualify for the 2016 Stanley Cup playoffs.

During their first-round matchup against Nashville, Theodore's time on ice slowly lowered due to errors as the team eventually lost in the first round.

[24] Upon arriving, he was assigned to the Golden Knights AHL affiliate, the Chicago Wolves, to begin the 2017–18 season,[25] but was recalled to the NHL on October 30.

During his time with the Golden Knights in the 2018 Stanley Cup playoffs, Theodore averaged 21:37 minutes and was praised for his skating ability by teammates.

[35] During the 2012–13 season, Theodore was a member of Team Canada under-18 squad when it won gold medals at both the 2012 Ivan Hlinka Memorial Tournament[36] and the 2013 IIHF World U18 Championships.

[37] Theodore would again play for the Team Canada during the 2015 World Junior Ice Hockey Championship, where he helped them hold the Russians scoreless in the third period to win the gold medal.

[38] On April 29, 2019, Theodore was slated to make his full international debut after he was named to the Team Canada roster for the 2019 IIHF World Championship, held in Slovakia.

[39] While undergoing preparations to compete in the tournament, he was diagnosed with testicular cancer[40] after failing a drug test due to high hCG hormone levels.

[41] In spite of this, he helped Canada progress through to the playoff rounds before losing the final to Finland to finish with the silver medal.

[1] His older sister Alyssa also plays ice hockey in women's and men's leagues while attending the University of the Fraser Valley for her Bachelor of Arts degree.