He stood for the Labour Party in the safe Conservative seat of East Fife in the 1970 general election, losing to the sitting MP Sir John Gilmour. He became Member of Parliament for Stirling and Falkirk at a by-election in September 1971, following the death of Malcolm MacPherson. His seat was renamed Stirling, Falkirk and Grangemouth for the general election in February 1974; despite swings to the Scottish Nationalists, he retained the seat in February and in October 1974, but with a much reduced majority. Harold Wilson appointed him as Under-Secretary of State for Scotland with responsibility for devolution and home affairs, retaining that position under Jim Callaghan as the Labour government pushed forward with a proposal for Scottish devolution. The eventual referendum in March 1979 failed to pass by a sufficient majority. The Labour government fell at the ensuing 1979 general election, although Ewing retained his seat with a substantially increased majority. He became a front-bench spokesman on Scottish industry in 1981. He came third in the ballot for chair of the Parliamentary Labour Party in November 1981. His seat was redraw again at the 1983 general election, and he was returned for the successor seat of Falkirk East, and he became a spokesman on trade and industry matters under Neil Kinnock. He was reelected in 1987, and stood down from his front bench position. He was a joint chairman with David Steel of the Scottish Constitutional Convention formed in 1989 to plan for the devolution of Scotland that was planned if Labour won the 1992 general election. Labour lost the election, and the plans were shelved. Ewing retired from the House of Commons at the 1992 general election.