Born in Abersychan, Monmouthshire in south-eastern Wales, the son of an NUM official and later MP, he was educated at Abersychan County School, University College, Cardiff[?], and at Balliol College, Oxford, where he took First Class Honours in Politics, Philosophy and Economics (PPE). University colleagues included Tony Crosland, Denis Healey, and Edward Heath. During World War II he served with the Royal Artillery[?] and then at Bletchley Park. He married Jennifer Morris in 1945.
He entered the British House of Commons in 1948 as representative for Southwark, having previously failed to win in Solihull in 1945. In 1950 he changed constituency to Stetchford[?], Birmingham and remained MP for there until 1977. He was Home Secretary from 1965 to 1967 and was responsible for the relaxation of laws over abortion, homosexuality, divorce and censorship. From 1967 to 1970 he was Chancellor of the Exchequer, replacing James Callaghan following the devaluation of the pound in November 1967. He quickly gained a reputation as a particularly tough chancellor, although he was hesitant about increasing taxes and reducing expenditure.
When Labour returned to power he was made Home Secretary again, serving from 1974 to 1976. Although he was tempted to challenge for leadership of Labour in March 1976 he instead was the first ever British citizen to be President of the European Commission, succeeding François-Xavier Ortoli[?], remaining in Brussels until 1981.
Jenkins split from the Labour party over policy and, as one of the so-called "gang of four", was a founder of the SDP in January 1981 with David Owen, Bill Rodgers and Shirley Williams. He led the new party from March 1982 until after the 1983 elections, he was a SDP MP for Glasgow Hillhead from 1982 to 1987.
From 1987, Jenkins remained in politics as a member of the House of Lords as Lord Jenkins of Hillhead. Also in 1987, Jenkins became Chancellor of Oxford University. In 1993, he was appointed to the Order of Merit. He was leader of the Liberal Democrats (UK) in the Lords until 1997. In December 1997 he was appointed chair of an independent commission, which became known as the "Jenkins Commission[?]", to consider alternative voting systems for the UK. The Jenkins Comission reported in favour of a mixed system called "limited AMS" or "AV Top-up" in October 1998. No action had been taken on this recommendation at the time of Jenkins' death from a heart attack in 2003.