Upon turning professional in 1950–51, Armstrong was assigned to Toronto's American Hockey League (AHL) affiliate, the Pittsburgh Hornets. In 71 games for Pittsburgh, he recorded 15 goals and 48 points. Despite being hampered by hand and wrist injuries suffered in fights, Armstrong was the AHL's leading goal scorer and stood second in points by mid-season in 1951–52. He was recalled to Toronto during the season and scored his first NHL goal, against goaltender Gerry McNeil of the Montreal Canadiens. It was the first goal ever scored by a player with Native heritage. He finished the season with three goals and three assists in 20 games with Toronto.