Educated at schools in Brandon and Melfort, Saskatchewan, MacEwan graduated from Ontario Agricultural College in 1926 and from Iowa State College in 1928. He was professor of animal husbandry at the University of Saskatchewan and department head from 1928 to 1946, and dean of agriculture, University of Manitoba, from 1946 to 1951.
He was the Liberal candidate in the federal by-election in Brandon in 1951, but was defeated. He moved to Calgary and was elected alderman in 1953, 1955, 1957 and 1959; he was mayor from 1963 to 1966. Elected MLA in 1955, he became Alberta Liberal leader in 1958, but was defeated in the election of 1959 and resigned the leadership in 1960.
Since 1936 he published four agricultural texts (two collaborations) and