Ewen Judson MacMillan was educated at Guelph Agricultural College, Ontario, and obtained the degree Bachelor of Science in Agriculture (BSA). From 1901 he was secretary for agriculture of Prince Edward Island, Canada. In June 1904 he was appointed to the Department of Agriculture of the Orange River Colony (now the Free State) as chief of the Experimental Farms and Livestock Division. In July 1905 he was appointed also as assistant director of agriculture for the colony, and in 1908 in addition succeeded M.S. Galbraith* as chief of the Agronomy and Seed Division. His post became that of chief agronomist in January 1909. After the formation of the Union of South Africa in 1910 he was appointed acting under secretary for agriculture in the Orange Free State province (now the Free State) in 1911.
During these years MacMillan wrote the department's Pamphlet No. 10 (1905/6) on "Silos and ensilage". He also contributed an article on "Winter feed for stock" to the Natal Agricultural Journal (1909, Vol. 13, pp. 209-215) and two years later one with the same title to the Agricultural Journal of the Union of South Africa (1911, Vol. 1). In October 1910 he furthermore published two items in the last issue of the Transvaal Agricultural Journal, on "Dry-land culture", and "The production of maze in the province of the Free State". He became a member of the Philosophical Society of the Orange River Colony in 1907.
In February 1913 MacMillan was appointed principal of the School of Agriculture at Potchefstroom (established in 1909). A few years later he reported on "Experiments with maize at the School of Agriculture and experimental farm, Potchefstroom" in the Agricultural Journal (Union of South Africa; 1917, Vol. 6). In 1919 he became under secretary for agriculture of the Union (at first in an acting capacity), with responsibility for education. He retired in 1922 and was appointed assistant district forest officer that same year.