He was educated at Paisley Grammar School, and the School of Law of the University of Glasgow, where he graduated with a First in Law. He held the Faulds Fellowship in Law at the University from 1965–1968, having been admitted to the Faculty of Advocates in 1967, and took a Doctor of Philosophy in 1969, entitled The rights and liabilities of the undisclosed principal in the law of agency.
From 1974-1976, McEwan was Standing Junior Counsel to the Department of Energy, and was appointed Advocate Depute in 1976, serving until 1979. He took silk in 1981, and became Chairman of the Industrial Tribunals the same year, serving until 1982, when he was appointed Sheriff of South Strathclyde, Dumfries and Galloway, first at Lanark but moving in 1988 to Ayr. In 1991, McEwan became a temporary judge of the Court of Session and High Court of Justiciary, Scotland"s supreme courts, and was appointed a permanent Senator of the College of Justice in 2000, taking the judicial title, Lord McEwan.
He sat in the Outer House.
He retired in 2008, his vacancy being filled by Valerie Stacey, Lady Stacey.
Faculty of Advocates]
He was a member of the Scottish Legal Aid Board from 1989 - 1996.