Initially he worked as a choirmaster in Glasgow and then at Lanark parish church, then moved to London, studying at the Royal Academy of Music with Frederick Corder, Tobias Matthay and Ebenezer Prout.
Later he taught for over a quarter of a century at the Royal Academy of Music in London as professor of harmony and composition, finally becoming principal of the Academy. He was also president of the Incorporated Society of Musicians (ISM) and co-founded the Society of British Composers.
He was very left-wing, politically, writing about abolishing money and total democracy.
McEwan died in London on 14 June 1948, aged eighty.