Born the son of Charles and Helen MacMillan in the township of Ayr, Scotland in 1741; James MacMillan was usually just referred to as Mac. Athletic in youth, his long flowing red hair caught the attention of many a maiden.

Immigrated with his family to America in 1753, but never truly left his beloved Scotland and the family farm. Enlisted with the Rangers as a teen and quickly became one of their best fighters. He is said to have saved Wade Stoners life. Throwing a tomahawk on a bead from 50 feet, he managed to split open the skull of a savage about to rip apart Wade's throat. "No one could throw a tomahawk like Mac, no one", proclaimed Dave Mackenzie. One of the few survivors of the raid on St. Francis.

Departed for Edinburgh Scotland in 1761 when Lord Thomas offered to pay for a college education at the University of Edinburgh. Graduating at the top of his class, Lord Thomas put him in charge of several of his Scottish estates.

Married Elizabeth Frazier in 1768, and had three daughters. Inherited the bulk of Lord Thomas's estate upon his death in 1781.

Died February 18, 1788 in Edinburgh, when the inn he was staying at caught fire and burned to the ground with 12 other victims. His oldest daughter Mary, inheriting the family fortune some years later, and not having any family of her own, liquadated the majority of the estate in 1845. Her intention was to feed the hungry and starving during the great famine in Scotland and Ireland. She was said to be moved by the generosity of her father and Lord Thomas, and the horrid story of starvation from the return of the St. Francis raid.