Davies was born in Llangennech, Carmarthenshire, Wales, the son of Rhys and Ann Davies. After an elementary education he initially worked as a farm labourer. He subsequently moved to the Rhondda Valley, where he worked as a coalminer for ten years. From an early age he was involved in the local co-operative society and became a union official organising shop-assistants throughout South Wales. This led to his moving to Manchester, to take up a post with the Amalgamated Union of Co-operative Employees and later the National Union of Distributive and Allied Workers.
In 1921, William Wilson, Labour Member of Parliament (MP) for Westhoughton, died, and Davies was elected at a by-election to succeed him. He served in the first Labour Government in 1924 as Under-Secretary of State for the Home Department. This was the only government office he held.
He retained the Westhoughton seat through successive elections until he retired from the House of Commons early in 1951 due to ill health. At the time he was the longest-serving Labour MP. Davies was a strong supporter of the temperance and pacifist movements, whose causes he advocated in Parliament and in the country. He was also a committed parliamentarian, and served as joint secretary of the British group of the Inter-Parliamentary Union.
- 'Obituary: Mr R J Davies', The Times, 2 November 1954.
- 'MP not to seek re-election', The Times, 10 April 1951, p. 7.