Mosdell started out with the Montreal Junior Royals in 1938-39, The next season, Mosdell scored 10 goals and 16 assists in 10 Memorial Cup playoff games. The Royals lost the 1940-41 Memorial Cup Final to the Winnipeg Rangers. The next season he was signed by, and played for the Brooklyn Americans in their last season, He spent the next two seasons playing senior hockey in the Montreal area. The Canadiens obtained his rights from the Americans. He played with the Habs from 1944-45 through 1955-56, usually as a defensive specialist. He won three Stanley Cups in 1946, 1953, and 1956.