MacMillan was born in Saint Croix in the Danish West Indies in December 1872. His father, Donald MacMillan, had been a paymaster with the Royal Navy and was later a sugar planter. His mother, Maria Elise Barca, was the Baroness de la Barca, the daughter of Baron de la Barca.
Following an uprising by workers in 1879, the MacMillans left Saint Croix. They were part of the George Vesey Stewart Special Settlement to Te Puke, however his father wanted to be closer to the sea and thus bought land at Katikati. They moved to Tauranga in 1908.
MacMillan was the 15th Mayor of Tauranga and served from 1915 to 1917.
In the 1915 mayoral election, he defeated John Cuthbert Adams. In the 1917 mayoral election, Adams narrowly defeated MacMillan (by 292 to 276 votes).
He was a cabinet minister in the United-Reform coalition government (Minister of Agriculture and Minister of Mines from 13 February 1932 to 6 December 1935). In 1935, he was awarded the King George V Silver Jubilee Meda
MacMillan died in Tauranga on 9 January 1941.
He was Mayor of Tauranga (1915–1917) and a member of the House of Representatives (1923–1935). He was subsequently granted the right to retain the title of "Honourable", having served more than three years as a member of the Executive Council.
