Armstrong played both junior and senior hockey in the Toronto Marlboros organization and was a member of the 1950 Allan Cup winning team as senior champions of Canada. He returned to the Marlboros following his playing career and coached the junior team to two Memorial Cup championships. He served as a scout for the Quebec Nordiques, as an assistant general manager of the Maple Leafs and for part of the 1988–89 NHL season as Toronto's head coach. Armstrong was inducted into the Hockey Hall of Fame in 1975 and the Maple Leafs honoured his uniform number 10 in 1998, and later officially retired the number, along with ten others, during a pre-game ceremony on October 15, 2016.