Having turned professional in 1991, King was ranked within the world's top 32 players between 1996 and 2015, and won his first ranking event title in 2016, defeating Barry Hawkins 9–8 in the final of the Northern Ireland Open. He has also appeared in two other ranking tournament finals: the 1997 Welsh Open, where he lost 2–9 to Stephen Hendry; and the 2004 Irish Masters, where Peter Ebdon defeated him 10–7.