Early itinerant, born at Killin, Perthshire. He went to sea at 14 despite parental opposition. Returning toEdinburgh in 1763 he joined the society, began to preach in 1766, was accepted as an itinerant and appointed to Newcastle. Returning after three years in Ireland he found the Edinburgh preaching house in urgent need of repair. Though recognising his ability and integrity, John Wesley judged him 'too warm and impatient of contradiction' and disapproved of his alienating Scottish Presbyterians by preaching against 'final perseverance'. In 1779, as Assistant in the Bristol Circuit, he fell foul of Wesley by challenging his invitation to the Rev. Edward Smyth to preach in the Bath chapel, but (in spite of Charles Wesley's implacable hostility) he had the support of most of his fellow itinerants and was reinstated at the 1780 Conference. Thomas Taylor commended his 'integrity and uprightness' and Thomas Rutherford spoke of his 'natural, simple oratory'. He left the itinerancy in 1783 and became pastor of a small Independent chapel in Sheffield.
John Wesley's Journal:
November 1779: 'Some time since Mr. Smyth, a clergyman whose labours God had greatly blessed in the north of Ireland, brought his wife over to Bath, who had been for some time in a declining state of health. I desired him to preach every Sunday evening in our chapel while he remained there; but, as soon as I was gone, Mr. McNab, one of our preachers, vehemently opposed that, affirming it was the common cause of all the lay preachers, that they were appointed by the Conference, not by me, and would not suffer the clergy to ride over their heads, Mr. Smyth in particular, of whom he said all manner of evil. Others warmly defended him; hence the society was torn in pieces and thrown into the utmost confusion. 'I read to the society a paper which I wrote near twenty years ago on a like occasion… In the morning, at a meeting of the preachers, I informed Mr. McNab that, as he did not agree to our fundamental rule, I could not receive him as one of our preachers till he was of of another mind… A few at Bath separated from us on this account, but the rest were thoroughly satisfied.'
McNab to Christopher Hopper, 2 February 1780:
'I strenuously remonstrated against to Mr. Wesley and firmly withstood Mr. Smyth; and threatened that if he would not desist from the Claim he had extorted from Mr. Wesley, I would complain to my Brethren the Preachers, whom I considered as being all concerned in the Cause. This opened a Field for a paper-war between Mr. Smyth and me. Mr. Wesley supported Mr. Smyth, and for that reason many Letters passed between him and me also. And my Fellow-labourers joined with me in Two Letters to him complaining of Mr. Smyth's extorted Permission. After Six or Seven Weeks Mr. Wesley, Mr. C. Wesley, Dr. Coke and Mr. Bradford came down Post-haste from London to Bath. Without speaking a Word to us the Preachers, Mr. Wesley that Evening in the Society utterly condemned us; and justified Mr. Smyth. The next morning he called us all together and in the presence of my friends excluded me from the Connection, and wouild not allow me to speak one Word in my own Defence.'
Quoted in John Lenton, John Wesley's Preachers (2009) p.296