Mainwaring was elected as the Labour MP for the Rhondda East division in a by—election in 1933 following the death of the sitting Labour MP, Colonel D. Watts—Morgan. But his election to parliament was by no means a foregone conclusion. He was opposed by Arthur Horner as a Communist candidate, and a Liberal also stood. Mainwaring's majority over Horner was just 2,899 votes, while the Liberal came in third. The constituency was a stronghold of Communism, and in each subsequent parliamentary election Mainwaring faced a Communist opponent. He twice defeated the redoubtable Harry Pollitt, although his majority in 1945 was no more than 972 votes. Strangely, in the general election of February 1950, when Pollitt stood again (for the last time) the margin between them was a massive 22,182 votes. He was opposed by Annie Powell as the Communist aspirant there in 1955. Mainwaring continued to represent Rhondda East until he resolved to retire from parliament in 1959. He often spoke in the House of Commons with passion and fire on behalf of his fellow miners. Mainwaring was notably well travelled. He served as a member of the Royal Commission on Rhodesia and Nyasaland in 1938. In 1943 he was selected by the Inter—Parliamentary Union to tour the East African territories, and he also spent four months in the USA undertaking an economic investigation. He toured Asia in 1956. A small group of his papers is in the custody of the National Library of Wales. A native Welsh speaker, he lived at 11 Aubrey Road, Penygraig in the Rhondda valley and also owned a home at 18 Harbord Road, Oxford. He married in 1914 Jesse, the daughter of Thomas Hazell of Oxford. They had one daughter. His wife had predeceased him. He died on 18 May 1971 aged 87. After a private cremation at Oxford his ashes were interred at Glyntaff Crematorium.