He was licensed by the Presbtery of St Andrews in August 1642, and became tutor to the eldest son of John Campbell, first Earl of Loudoun, the Lord High Chancellor of Scotland.
Some persons from Fenwick having heard him preach at a Fast-day service in Galston Kirk, they desired him to be called as the first minister of their newly created parish.
Soon afterwards the General Assembly appointed him an army chaplain, and he was present at the engagement which took place at Mauchline Moor in June 1648.
He went to Pitforthie, which had again come into his hands through the death of his brother, and following a period of ill-health, he died in the manse of his brother-in-law, Laurence Skinner, minister of Brechin, 10 October 1665, and was buried in the Cathedral there.
[3] Hew Scott says he was a man of ready wit, who moved freely amongst his people, lived simply, and spent a great part of his leisure in such sports as fishing and fowling.
Guthrie's book, The Christian s Great Interest (which has been translated into several foreign tongues), was published to vindicate himself against a volume purporting to contain a series of his sermons on Isaiah 55.
In 1680 another work professing to be his was issued, with the title The Heads of some Sermons preached at Finnick in August 1662, but this also was disclaimed by his widow in a public advertisement.
lord Boyd of Kilmarnock, patron of the parish, a strong loyalist, opposed the choice, but Guthrie was ordained at Fenwick by Irvine presbytery on 7 November 1644.
A ready wit and unconventional dress earned him the appellation of 'the fool [jester] of Fenwick,' which appears even on title-pages of his sermons.
Finding that one of them went fowling on Sunday, and made half-a-crown by it, he offered him that sum to attend the kirk, of which the man ultimately became an elder.
At the Restoration he was prominent in his efforts for the maintenance of the presbyterian system, proposing at the synod of Glasgow and Ayr (2 April 1661) an address to parliament for protection of the liberties of the church.
[4] In August 1645 he married Agnes (who survived him), daughter of David Campbell of Skeldon House in the parish of Dalrymple, Ayrshire.
This book, which is based on sermons from Isaiah lv., has passed through numerous editions (e.g. 4th edition, 1667, 8vo; Glasgow, 1755, 8vo; Edinburgh, 1797, 12mo), and has been translated into French, German, Dutch, Gaelic (1783, 12mo, and 1845, 12mo), and ‘into one of the eastern languages, at the charge of the honourable Robert Boyle.’ Its publication was occasioned by the issue of a surreptitious and imperfect copy of notes of the sermons, issued at Aberdeen, 1657, with the title ‘A Clear, Attractive, Warming Beam of Light,’ &c. In 1680, 4to, appeared ‘The Heads of some Sermons preached at Fenwick in August 1662, by Mr. William Guthrie;’ his widow, by public advertisement, disclaimed this publication as unauthentic.