Duncan was the 13th head football coach at Appalachian State Teachers College—now known as
Appalachian State University—located in the town of
Boone, North Carolina, serving from 1960 to 1964. He had a 31–15–2 as the
Mountaineers head coach. On December 4, 1964, Duncan resigned as head football coach at Appalachian State. In 1965, Duncan joined the
Saskatchewan Roughriders of the
Canadian Football League (CFL) as an assistant under head coach
Eagle Keys. He was with the team when they defeated the
Ottawa Rough Riders in the
54th Grey Cup and when the team lost to the
Hamilton Tiger-Cats in the
55th Grey Cup. Duncan became head coach of the
Calgary Stampeders in 1969, replacing
Jerry Williams who left the team to join the
Philadelphia Eagles. Duncan's stint with the Stamps resulted in two
Grey Cup appearances; one win (
59th) and one loss (
58th). Duncan was fired in 1973 after back to back 6–10 seasons. His overall record with Calgary was 39–40–1. Duncan was later hired by a group from
London, Ontario, who hoped to bring professional football to their city, and later served as executive assistant of the
Portland Storm of the
World Football League (WFL). ==Personal life==