Born at Barncauchlaw, Minigaff, Kirkcudbrightshire, in 1669?, and spent his boyhood near his birthplace. He appears to have been raised among the United Societies from his youth, but entered the Revolution Church for a time. Before he began his ministerial career he was elected an elder of the Girthon session. He attended Edinburgh University 1695-7, and graduated M.A. on June 28, 1697. He was licensed on November 26, 1700, spending part of his probation as tutor with the Laird of Broughton, 1700-1. He preached for the first time in Balmaghie Church on December 22, 1700, apparently as ordinary supply. On April 30, 1701, he was elected to the parish. The call was reported to the Presbytery on June 24, and he was ordained on September 19. The controversy regarding his ecclesiastical attitude lasted from October 1702 to December 30, 1703, when he was deposed. His name first appears in the minutes of the General Meeting of the United Societies (Covenanters) when they considered a letter from him, April 5, 1704. He conferred with its members on January 31, 1705 and February 13, 1706, and on August 14, 1706, submitted to them. The Societies called him on October 9. The Covenants were renewed at Auchensaugh on July 23-4, 1712. M'Millan left the Balmaghie manse in 1727, and during 1729-34 resided at different places in the parish of Carnwath, and at Braehead from 1743-53. The Presbytery was erected at Braehead on August 1, 1743, and a disruption took place in it April 1753. He was the first minister among the Cameronians (Covenanters) after the Revolution Settlement. From 1706 until 1743, when Presbytery was erected, he was the only ordained minister among the Covenanting Societies. For part of this time he was assisted by Mr. John M'Neil, a probationer and preached of the Gospel. He died at Broomhill on December 1, 1753.