Educated at Balliol College, Oxford, from which he graduated in 1941, Jenkins served in the Royal Artillery in World War II and first entered Parliament in 1948. He could claim family roots in the Labour movement; his father had been a miners' union official, a member of Parliament, and parliamentary private secretary to the Labourite prime minister Clement Attlee. Jenkins at one time considered giving up politics for writing, but, in the formation of the 1964 government of Harold Wilson, he joined the cabinet as air minister (1964–65); he then became home secretary (1965–67) and chancellor of the Exchequer (1967–70). In 1972 he resigned from the Labour Party in protest of its decision to support a referendum on whether Britain should remain in the Common Market. He reentered the shadow cabinet in 1973 as shadow home secretary and became home secretary after Labour's victory in 1974. In 1976 he resigned from the cabinet and Parliament to become president of the executive branch of the European Community, and he remained in that post until 1981. In 1981 he and other dissidents from the increasingly leftist Labour Party formed the Social Democratic Party, of which he was briefly leader. In 1987 he accepted a life peerage and moved from the House of Commons to the House of Lords, where he was a leader of the new Social and Liberal Democratic Party (Liberal Democrats). He subsequently became chancellor of the University of Oxford (1987–2003). In 1993 Jenkins was elected to the Order of Merit.

Jenkins wrote numerous books, including biographies such as Asquith: Portrait of a Man and an Era (1964), Baldwin (1987), Gladstone (1995), and Churchill (2001), and political works such as Mr. Balfour's Poodle: Peers vs. People (1954), The Labour Case (1959), and Afternoon on the Potomac?: A British View of America's Changing Position in the World (1972). A Life at the Centre: Memoirs of a Radical Reformer (1991) recounts Jenkins's own political career.