He turned professional in 1980, and surprisingly defeated the defending champion Steve Davis 10–1 in the first round of the 1982 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.

[1] He began playing snooker at the age of 9 on the tables at the Tonge Moor Conservative Club, which was run by his father, Kevin.

[5] He reached the quarter-finals of the 1981 UK Championship by eliminating Geoff Foulds, Fred Davis and Doug Mountjoy from the competition before being defeated 5–9 by Terry Griffiths.

[5][6] Later that season, Knowles gained attention when he won 10–1 against defending champion Steve Davis in the first round of the 1982 World Snooker Championship, after staying out late at a nightclub following the first day's play when he had built an 8–1 lead.

[1][7] In the 1982–83 snooker season, he followed up his performance in the World Championship by winning the 1982 International Open with a 9–6 victory against David Taylor.

[5] Having failed to successfully defend the International Open title, losing 4–5 to John Spencer in the second round, Knowles started the 1983 Professional Players Tournament with a 5–1 win against Paul Medati and a 5–4 defeat of Rex Williams, then a 5–0 whitewash of Silvino Francisco.

[5] The same day as his match against Francisco, Knowles won 5–3 against John Campbell in the quarter-finals, after losing the opening two frames.

[14] At the start of the 1984–85 snooker season, he won the 1984 Australian Masters by defeating John Virgo 7–3 in the final, and was the runner-up, 7–9 to Jimmy White, at the 1984 Carlsberg Challenge.

[18] At the 1991 Dubai Classic, he defeated Gary Natale 5–1 in the qualifying competition, then Eugene Hughes 5–2, and Neal Foulds 5–0.

Facing reigning world champion Parrott in the final, Knowles fell 0–3 behind, but won three of the next four frames to trail 3–4 at the end of the first session.

[22][23][24] He finished tenth in the UK Tour standings, which meant he regained his full professional status for 1998–99 snooker season,[25] losing it again at the end of 2000–01.

[29] He failed to regain his professional status, but reached the last 32 of event 3 of the 2021 Q School, where he lost 2–4 to Mark Lloyd.