McMillan was born in Rigaud, Lower Canada, the son of Donald McMillan and Olympe Mongenais. He served on the county council and was a captain in the local militia. In 1862, he married Agnes Mongenais, the daughter of Jean-Baptiste Mongenais, who was then representing Vaudreuil in the Legislative Assembly of the Province of Canada. From 1865 to 1873, McMillan ran the grain and feed business established by his father at Rigaud, later building and operating a sawmill there.The Canadian parliamentary companion, 1889, JA Gemmill He died in Rigaud at the age of 55.
