Born in the parish of Carnwath, Lanarkshire, in 1729. While a student he supported himself by teaching. He first appears in minutes of Presbytery in 1760. After a long examination, extending over many months, he was licensed on February 21, 1761. In the same year he was sent to Ireland, and while there was presented with a call from the vacant congregation of Londonderry, Donegal and Tyrone. He accepted it, but the ordination was not carried out, and in April 1762 he was recalled to Scotland. At the next meeting of Presbytery he was appointed clerk and dropped the call. At the meeting of August 1763 he was offered two calls—one from America and the other to be the colleague to John Courtass in the Southern congregation. He accepted the latter, and was ordained at Leadhills on December 21, 1763. At first he resided at Thirton House near Douglas, and afterwards at Howgill, Newtonhead. A church was built for him at the former place. His people were scattered over the southern Scottish counties, but in his last years he confined his labours to the near neighbourhood of his home. He died on April 18, 1806. Fairley was one of the "four Johns" of the Reformed Presbytery. He is most noted for his work defending the Reformed Presbyterian position on civil magistracy against the detractions of the Seceders, An Humble Attempt in Defence of Reformation Principles, on the Head of Civil Magistracy, published 1770.