In 1962, Armstrong's professional coaching career began when he was hired as an assistant coach with the American Football LeagueHouston Oilers. After serving two years in that capacity, he shifted back to Canada as head coach of the Edmonton Eskimos. In his six years, the team reached the postseason three times.
Armstrong was hired as an assistant with the Minnesota Vikings in 1970, and became an integral part of developing the team's dominating defense. After helping the team reach the postseason in all but one of the next eight years, he was hired as head coach of the Chicago Bears on February 16, 1978.
In four years at the helm of the Bears, he was only able to compile a record of 30-35, with one playoff appearance in 1979. He was fired on January 3, 1982, but hired less than two months later as an assistant with the Dallas Cowboys. He spent the next eight seasons with the team before announcing his retirement on February 22, 1990.
He and Bud Grant hold the distinction of being the only two people to have both played and been a head coach in both the NFL and CFL.