Thomas was raised by his mother in the village of Trealaw in South Wales, just across the river from the town of Tonypandy. All four of his siblings left school at age 13. His two sisters went into service, his elder brother went down the pit and his younger brother worked in a shop. This allowed George to extend his education, a good education at the time being the best means of escape from the valleys. He attended Trealaw Boys' School where he passed the scholarship examination for Tonypandy Higher Grade School, later promoted to Tonypandy Secondary Grammar School.
On leaving school Thomas became a pupil teacher, first in Trealaw and then in Fanshawe Crescent School, Dagenham, Essex, after which he did a two-year teacher-training course at University College, Southampton. He then worked as a teacher in both London and Cardiff.
Thomas was one of the first on the scene of the Aberfan disaster, which occurred while he was a Minister at the Welsh Office. Thomas showed sympathy with the people of the village, bereaved and devastated by the calamity which cost the lives of 144 people, 128 of them children at the Pantglas Junior School. However, he subsequently insisted on taking over £150,000 from the charity fund established to assist the bereaved, in order to meet the cost of removing the remainder of the coal tip and the remaining tips which still loomed above the village of Aberfan. This money was susbequently returned to the fund in 1997, by the then Welsh Secretary Ron Davies, and even then returned without any financial interest.
During Thomas's term of office as Speaker of the House of Commons from 1976 to 1983, the first broadcasting of Parliamentary proceedings brought him unprecedented public attention, but he proved more impartial than party colleagues had expected. In 1983 he retired and was created Viscount Tonypandy, one of the last creations of a hereditary peerage. Thomas was always opposed to Welsh nationalism: one of his final areas of political activity was the public expression of opposition to the Blair government's devolution proposals of 1997. It was during this year that he also gave his very high-profile endorsement of Sir James Goldsmith's Referendum Party, believing that the European Union was compromising the sovereignty of Parliament. He also wrote the Foreword to Adrian Hilton's book on this issue, The Principality and Power of Europe, the only book he endorsed as a Peer, and the last before he died.
After Tonypandy's death, a former Welsh Labour MP, Leo Abse, created a controversy by alleging that Thomas had been homosexual and had been the victim of blackmail for this reason. Abse, the MP who introduced the private member's bill which decriminalised homosexuality in Britain, discussed this incident in his book Tony Blair: The Man Behind the Smile. He said that Thomas had paid money to blackmailers to keep information related to his sexual life secret. Abse said that he had once lent Thomas £800 to pay off blackmailers.
Throughout his career he remained a deeply religious man, and was a prominent member of the Methodist church. He was a local preacher and former Vice-President of the Methodist Conference. Known by the nickname "Tommy Twice" (from his full name), his Welsh-accented cries of "Order! Order!" as Speaker were familiar to a generation of Britons.