Thomas was elected to Parliament as MP for Conway in 1951,[7] winning a narrow majority in the marginal seat over the Labour incumbent.

He turned down the position of Under-Secretary of State for Wales at the Home Office to concentrate on his legal career but later served as Parliamentary private secretary to Sir Harry Hylton-Foster (the Solicitor General and later Speaker) from 1954 to 1959.

He served as Parliamentary Secretary at the Ministry of Labour 1959–61, taking charge of the measures that abolished the requirements for employees to be paid in cash and the maximum wage for a professional footballer (£14 per week in November 1960).

He moved to become Under-Secretary of State at the Foreign Office in 1961, travelling to Moscow with Lord Home in 1963 to sign the Nuclear Test Ban Treaty.

He was promoted to Minister of State for Foreign Affairs in 1963, and was sworn of the Privy Council in the Queen's Birthday Honours of 1964,[8] but left office when his party lost the 1964 general election.

He retired from the House of Commons at the 1987 general election, and was raised to the peerage for life in the Dissolution Honours that year,[18] gazetted as Baron Thomas of Gwydir, of Llanrwst in the County of Gwynedd.