 He joined the Labour Party in 1970, securing election as the Labour member for the Saron ward on the Dyfed County Council where he served from 1977 until 1979.In 1979, Thomas was elected Member of Parliament for Carmarthen, defeating the President of Plaid Cymru, Gwynfor Evans.It was one of only two Labour gains throughout the whole of Great Britain in that general election which saw Margaret Thatcher and the Conservatives sweep to power at Westminster.