Dumbarton born outside right Scott Duncan began his football career playing with Dumbarton Oakvale and Dumbarton Corinthians in 1903, and played for Clydebank Juniors in 1904 and Shettleston in 1905 before joining hometown Scottish League club Dumbarton in June 1906, making his Scottish League debut at St Bernard’s that August. Having established himself in their first eleven he scored twice in 32 appearances before he was signed by First Division Newcastle United for a fee of £200 in March 1908, making his Football League debut at Everton the following month. A versatile forward, although primarily an outside right, he scored 12 times in 81 appearances for The Magpies and was a member of their 1908-09 League Championship winning side. Although Newcastle reached both the 1910 and 1911 FA Cup Finals (winning the Cup in 1910), Duncan only played in early round ties each campaign.
In May 1913 he returned to Scotland, moving to Glasgow Rangers for a fee of £750. He remained with Rangers during the First World War, in which he served as a signalling instructor in The Royal Field Artillery, scoring 26 goals in 103 appearances. He also played two wartime games for Celtic while on loan in January 1919 and was also loaned to Partick Thistle. In 1919 he rejoined Dumbarton scoring twice in 30 further appearances before he signed for Cowdenbeath in the 1920 close season. In 1922 he rejoined Dumbarton for a third time, but retired at the end of the 1922-23 season without having played any further first team games for The Sons.
He began his career in football management with Hamilton Academical in 1923 and remained with them until he became manager of Cowdenbeath in October 1925. He had seven successful years at the club and kept Cowdenbeath in Division One throughout his tenure before becoming manager of Second Division Manchester United in October 1932. Despite spending a lot of money on players, United were almost relegated to the Third Division in 1933-34. However, Duncan turned things around and led United to the 1935-36 Second Division Championship. However, they were relegated the following season and in November 1937 he resigned to become manager of Ipswich Town, who were then a Southern League side.
At the end of the 1938-39 season Ipswich were elected to the Football League despite finishing third in the Southern League. Duncan remained with Ipswich Town as their manager for 18 years, winning the Third Division (South) Championship in 1953-54. However, Town were relegated the following season and Duncan stood down as manager in August 1955 when Alf Ramsey took over. He remained as the club’s secretary for three further years. He was honoured with a testimonial match against Norwich City at Portman Road at the end of the 1957-58 season, which Ipswich won 3-1.