Thomas was educated at Abertillery Grammar School and King's College London, where he was president of the Students Union 1955-56. He was a barrister, called to the bar by Gray's Inn in 1957, and was appointed Queen's Counsel.
Thomas was elected as a Labour Member of Parliament for Abertillery in 1970. In December 1981, he was one of a number of Labour MPs who defected to the new Social Democratic Party. His seat was abolished by boundary changes in 1983, and he stood in Cardiff West. He came third with 25.5% of the vote, which may have contributed to the victory of the Conservative Stefan Terlezki in a normally strong Labour seat.
He later rejoined the Labour party. He died in Pontypool aged 55.