Knowles was the British under-19 snooker champion in 1972 and 1974. He turned professional in 1980, and surprisingly defeated the defending champion Steve Davis 10-1 in the first round of the 1981 World Snooker Championship. In 1984, tabloid stories about his personal life were published, and he was fined £5,000 by the World Professional Billiards and Snooker Association for bringing the game into disrepute. His other tournament victories included the 1984 Australian Masters and, as part of the England team with Davis and Tony Meo, the 1983 World Team Classic.