Duncan began his schoolboy career with Belview Boys before joining St James's Gate in the 1910-11 season.
[citation needed] Together with Charlie Dowdall, Thomas Murphy, Ernest McKay and Michael Farrell, Duncan was one of five players from St James's Gate, included in the Irish Free State squad for the 1924 Olympic Games.
[1] Duncan's League of Ireland career was ended by injury in 1928 and a testimonial game was arranged for his benefit between Bohemian F.C.
Other members of Olympic squad included Tommy Muldoon, Joe Kendrick, Jack McCarthy, Johnny Murray and Dinny Hannon.
[3][4] The following day, before returning home, the team played Estonia in a friendly at the Stade Olympique and Duncan scored the opening goal in a 3–1 win.