Born at Eastforth, Carnwath, on July 4, 1729. He was numbered among the "four Johns" of the Reformed Presbytery. He was ordained at Bothwell on September 20, 1750. He first appears in support of his father against the heresy of James Hall and Hugh Innes in 1753. He probably was the author of the vindication issued afterwards entitled, A Serious and Impartial Survey (1754). He had no fixed residence at first, but it is probable that he lived for some time at Pentland before he went to Sandhills, near Glasgow, about 1786. He there took up house in a building belonging to some relatives. M'Millan itinerated among the scattered societies for many years. When he reached his jubilee, in 1800, he wished to retire, but his brethren were averse from the proposal, and he withdrew it. He died on February 11, 1808.