Thomas' first professional bout was at welterweight, fought in Manchester against Ray Farell, himself a fairly novice boxer with only two professional matches to his name. Thomas won by knockout in the second round, and this started a nine match unbeaten run, though none of his seven wins after Farell coming through knockout. His first defeat was a points decision to Alan Reid at the end of 1971, a fighter who he had faced and beaten six months earlier.
Thomas' next bout was in 1972, against Jimmy Fairweather in Walworth, saw Thomas now fighting in eight round matches. A win over Fairweather was followed by victories over Mickey Flynn and Phil Dykes, before being disqualified in an encounter with Charlie Cooper. At the start of 1973, he beat Des Rea, the Irish Welterweight Champion by a points decision; this was followed by two more wins before he faced Rea again, this time beating him by technical knockout in the fourth round. 1973 ended just like the previous two years, with a defeat, a narrow points loss to Trevor Francis at Sophia Gardens in Cardiff. The next year saw Thomas begin one of the most successful periods of his career, going on a twelve match unbeaten run. On 15 December 1975, Thomas faced Pat MacCormack for the vacant British Welterweight title. He knocked MacCormack down twice, before winning by knockout in the thirteenth and in the process became the first immigrant fighter to win the British Welterweight title.
After a win over Jim Devanny in February 1976, Thomas fought Italian Marco Scano in Cagliari, for the vacant European Welterweight title. Thomas was outclassed losing by knockout in the second round. This was followed by Thomas' second overseas match, facing Jørgen Hansen in Denmark; Thomas lost by technical knockout in the third. Back in Britain, Thomas successfully defended his welterweight title against Trevor Francis, but then lost the belt when he was knocked out by Henry Rhiney in Luton.
After losing the welterweight title, Thomas went up a weight division, now fighting at light-middleweight. He began in the new division by taking the Welsh Light-middleweight title from Dave Davies, who had beaten Thomas three times in the amateurs, in early 1977. Despite the encounter being for a Welsh title, the match took place at The Stadium in Liverpool, the fight being part of the undercard for John Conteh's WBC World Championship bout with Len Hutchins. Thomas' light-middleweight career was far more chequered than his time as a welterweight, losing to Larry Paul, before beating Rhiney and then drawing with Tony Sibson. In 1978 Thomas spent much of his time fighting outside Britain, but with little success. He fought twice in Netherlands, a win and a loss, which were followed by three straight loses, against former Olympic Lightweight Champion Chris Clarke in Canada, then French Champion Claude Martin and finally Spanish fighter Andoni Amana in Bilbao.
His fortunes changed on British soil, with wins over Salvo Nucifero and Robbie Davies, before losing to Kenny Bristol in a fight for the vacant British Empire Light-middleweight title by a unanimous points decision. The very next fight was another title decider, this time for the British Light-middleweight belt on 11 September 1979. In that encounter he beat Jimmy Batten in the ninth round to take his second British title; though Batten later stated that he felt he had given the title away as he wasn't in the correct frame of mind due to personal problems. Thomas defended his title successfully twice, knocking out Dave Proud in the seventh and then stopping Steve Hopkin in the final round with a badly cut eye. His third defence ended in defeat, losing to Hensol Graham by points. This was the start of a six-round losing streak that only ended with his retirement from boxing in 1984.