After the war, he resumed his faculty position at the rank of Associate Professor and in 1923 was appointed Chair of the English department.
His entry to politics was through appointments to the federal Royal Commission on Maritime Claims (1926) and Atlantic Fisheries (1928).
From 1943 to 1946, he was the Parliamentary Assistant to the Minister of National Defence for Air and served on or chaired numerous government committees.
It was completed in 1909, comprising four volumes in English and in French and more than a thousand pages of text, both in type and, in the case of many of the song lyrics, handwritten.
In addition to playing a major role as a professor of English in establishing McGill's theatre program, he served as Dean of Arts and Science from 1940 to 1947, and he published books on Canadian mythology.
His letters home to his wife, Margaret Eaton Brower (they had been married in Montreal in August 1916, just a month before he left for his overseas posting), and family offer a sanitized but beautifully written account of his war experiences.
Just prior to entering the theatre of war in France, MacMillan and Brower, herself a graduate of McGill University, made a brilliant marriage.
During the 1920s he was appointed to Royal Commissions for Maritime Claims, Atlantic Fisheries, and Prince Edward Island Education.