Born in Chiltern, Victoria, McEwen was a World War I soldier settler who became a dairy farmer in the Goulburn Valley district of Victoria. He was actively involved with the Victorian Farmers' Union and the Country Party during the 1920s and entered the federal parliament in 1934.

He won the deputy leadership of the parliamentary Country Party in 1943 and the leadership in 1962. As deputy Prime Minister from 1958, McEwen was acting Prime Minister many times.

McEwen was a minister in the Cabinets of five heads of government. He was Minister for the Interior (1937-39) in the governments of Joseph Lyons and Earle Page, Minister for External Affairs (1940) in Robert Menzies' government, and Minister for Air and Civil Aviation (1940-41) in Menzies' and Arthur Fadden's governments.

McEwen was a minister for the whole of Menzies' second term as Prime Minister from 1949 to 1966, serving as Minister for Commerce and Agriculture (1949-56), Minister for Trade (1956-63) and Minister for Trade and Industry (1963-66).

He was also Minister for Trade and Industry (1966-67) in Harold Holt's government, retaining the portfolio when he succeeded Holt as Prime Minister, and from 1968 to 1971 in the government of John Gorton. In 1956 he re-organised his portfolio into the Department of Trade and used this as a base from which to promote Country Party policies on primary and secondary industry within the government.

From 1958 he was leader of the Country Party and Deputy Prime minister.

On the sudden death of Harold Holt in December 1967 he became caretaker Prime Minister for three weeks until the Liberals chose John Gorton as their new leader.

Though only briefly Prime Minister, McEwen served as deputy Prime Minister for twelve years, in the governments of Robert Menzies, Harold Holt and John Gorton. He was acting Prime Minister many times in the years from 1958 to 1971.

McEwen held the key ministerial responsibilities of Commerce and Trade for 20 years from 1949 to 1971. During this time the portfolio emphasis moved away from agriculture and into the broader industry area. Though a farmer and Country Party leader, McEwen also moved the emphasis from low tariffs to protection of manufacturing industries.

He retired from politics in 1971 and was knighted the same year.