Duff started playing darts when he was almost 30 years old and did not turn professional until he approached 50.

[2] Duff reached the quarter-finals of the 2019 World Masters but lost to eventual champion John O'Shea.

He beat Richard Veenstra 5–2 in the semi-finals to reach the final, where he defeated Thibault Tricole 6–5 and became the first Northern Irish world darts champion, as well as the first player to win at the Lakeside on debut since Christian Kist at the 2012 BDO World Championship.

[7] Duff's win was celebrated at an event in his home town Ballyclare that was organised by the Antrim and Newtownabbey Borough Council.

[8] At the 2023 WDF World Championship, Duff began his title defence with a 3–1 win over Moreno Blom.