Thorne became national under-16 champion at snooker in 1970. He never really converted this early promise into professional success, however, only ever winning one ranking snooker tournament (the Classic in 1985). The same year, he reached the UK Championship final against the then dominant Steve Davis, and seemed to have built himself an unassailable 13–8 lead. But a miss on a straightforward blue off its spot during the first frame of the final session allowed Davis to take the frame and eventually win the title. Thorne later said that he had hardly looked at the blue, considering it a certainty. He reached the quarter-finals of the World Snooker Championship in 1982 and 1986.
He peaked at no. 7 in the rankings in the mid-1980s, while also battling a serious gambling problem. In one famous incident, Thorne bet £38,000 on a match involving John Parrott, betting that Parrott would lose as Parrott had lost his personal cue and had to use one supplied by the venue. Much to Thorne's dismay (not least because he was actually commentating on the match), Parrott recovered from a slow start to win, only worsening Thorne's debts. Also, in an interview with The Guardian newspaper in 2004, Thorne admitted to placing bets worth up to £20,000 on a horse.
Thorne's bald head makes him instantly recognisable and he is often referred to as the "Homer Simpson of Snooker". He has become a popular senior character in the game, commentating on snooker for television on the BBC and Sky Sports. Alongside other Matchroom professionals, Thorne featured in the popular song "Snooker Loopy", written and performed by Chas & Dave. In the verse which begins "Willie Thorne, his hair's all gone", Willie's cameo line was "Perhaps I ought to chalk it", in reference to his gleaming head putting off his opponents. Willie also appeared in the "Romford Rap" video with the rest of the "Matchroom Mob".