"[2] Unable to make a living from snooker in Canada, Thornley travelled to the United States to play pool for money.

He turned professional in 1979, playing in three tournaments during the 1979/1980 season; in the Canadian Open, he lost to Bill Werbeniuk in the last 16, but having won two matches to progress to that stage, he did not win another, as Jim Wych eliminated him from the Canadian Professional Championship 9–2, and his 1980 World Championship campaign ended immediately, with a 4–9 loss to Australian Paddy Morgan.

Thornley played only one match in the ensuing three years – in the 1980 Canadian Open – but following his hiatus, he returned to compete in the 1984 World Championship.

The 1984/1985 season heralded no progress for Thornley, but in the next, he defeated Derek Mienie of South Africa 10–3 in qualifying for the 1986 World Championship, following this with a 10–7 victory over Patsy Fagan, to reach the last 64.

He lost this match 3–10 to Northern Ireland's Tommy Murphy, but earned £1,200 for his effort, and finished the season with a ranking position for the first time, at 97th.