MacMillan was born in Upper South River in Antigonish County.

He made his fortune in lumbering and construction before being made chairman of the Nova Scotia Highways Board in 1920 and serving briefly as minister of highways in 1925.

He was a member of Nova Scotia's appointed upper house, the Legislative Council from 1925 until 1928 when he won a seat in the Nova Scotia House of Assembly as a Liberal.

He retired as premier and from politics in 1945 to allow Macdonald to resume his provincial career.

This article about a Member of the Nova Scotia House of Assembly is a stub.