In 1793, having left Mr Neilson’s shop, he was found qualified to assist his elder brother in the management of a school, where writing, arithmetic, and mathematics were taught. Of this seminary he afterwards became sole master; but he ultimately abandoned teaching as a pursuit not agreeable to his genius or temper, and in 1798, was engaged as book-keeper and clerk by Mr David Lamb, an American cotton merchant, to whose two sons he at the same time acted as tutor. Some years afterwards, on Mr Lamb removing to America, Mr M’Gavin became his partner; the business was carried on in Glasgow. In 1805, Mr M’Gavin married Miss Isabella Campbell of Paisley. As his business was of a light nature, and Mrs M’Gavin brought him no children, he enjoyed more leisure for the cultivation of his mind than falls to the lot of most merchants in the busy capital of the west of Scotland. At a later period, after the death of his original patron, he entered into partnership with the son of that gentleman, and carried on what is called a West India business under the firm of M’Gavin and Lamb. This ultimately proving unprofitable, he was induced, in 1822, to undertake the Glasgow agency of the British Linen Company’s bank, which he conducted without intermission till his death.

Mr M’Gavin was brought up by his parents in the strictest tenets of the presbyterian faith, as professed by the congregations of original anti-burghers. About the year 1800, a conscientious dissent from the views of this body respecting church government induced him to join the Rev. Mr Ramsay in the formation of an independent or congregational church. In this communion he began to exercise a gift of preaching, with which he was endowed in a remarkable degree, receiving from Mr Ramsey the ordination which was considered necessary for the pastoral office by this body of Christians. Eventually, circumstances so much reduced the society, as to make it cease to answer what he conceived to be the design and use of a church—namely, "not only the edification of its own members, but the public exhibition of their spirit and practice, for manifesting the glory of the grace of God, and promoting the salvation of men." For this reason, in 1808, he joined the kindred congregation of Mr Greville Ewing in the Nile Street meeting-house, Glasgow, where he was soon afterwards invested with the office of deacon. Here he might have also continued to preach, if he had been willing; but he was now unable, from the pressure of business, to give the duty that attention which he deemed necessary, and accordingly resisted Mr Ewing’s frequent and urgent solicitations, though he occasionally consented to perform public worship in the neighbouring villages, or in places where he thought such ministrations eminently necessary.

Being a man of uncommon industry, and equally great benevolence, Mr M’Gavin found time, amidst his numerous mercantile avocations, to write a number of religious tracts and stories, for the improvement of the poorer and junior classes of society. Though these productions are of a class which do not usually attain a high place in literature, the reader, however indifferent to the subjects, or of however highly cultivated intellect, could peruse them, without remarking the extraordinary conciseness of style and moral force by which they are characterized. The most distinguished of all Mr M’Gavin’s writings is his "Protestant," a series of papers, designed to expose the errors of the church of Rome, commenced in 1818, and finished in 1822. In the general decline of religious controversial writing, the celebrity acquired by this work, is a strong testimony to the powers of the author. In its collected form, in four volumes octavo, it went through no fewer than seven editions in the first ten years. According to Mr Greville Ewing, in a funeral sermon upon Mr M’Gavin, "the commencement of the work was casual, and the whole executed with hasty preparation. While engaged in a mercantile business of his own, he had at that time the winding up of an old concern of his partner, the heavy charge of another concern, which in the end proved a severe loss to him, besides other business matters, as factorships, references, as sole arbiter, in cases both from private parties and from The Court of Session, which he decided in a manner satisfactory to all concerned; and many other things were devolved on him, which none but a man of clear judgment, and unusually industrious habits, could have undertaken. A work which, otherwise, would have been extremely irksome, was rendered pleasant by the continued and increasing favour with which it was received by the public in general, and by the approbation of distinguished individuals in each of the three kingdoms. One of the most eminent bishops of the church of England offered to give him holy orders. That, however, which was most gratifying to the author, was the interest which he was honoured to excite in the public mind, with regard to the subject of popery. I make no attempt to give a particular account of the contents of this work. It is impossible, they are so extensive: it is unnecessary, they are so generally known. It is matter of notoriety, that Mr M’Gavin was prosecuted for certain articles in the Protestant, and had a verdict against him, imposing on him a fine of £100, which, with expenses, amounted to above £1200. Into the merits of these things I shall not enter, further than to state, in round numbers, that £800 of the £1200, was raised by public subscription, and that the whole, it was believed, would have been more than paid, had not each subscriber been limited to a certain sum. As the case had been so arranged, Mr M’Gavin was obliged, in the mean time, to pay the balance out of his own pocket; of which, great as the amount was, I never heard him complain. The publishers afterwards very handsomely came forward to reimburse the author, which, from the sale of the work, they were enabled to do without loss to themselves, though he had no claim upon them."

Mr M’Gavin, in 1827, superintended a new and improved edition of "The Scots Worthies," a work commemorating the lives of the most eminent Scottish clergy of the sixteenth and seventeenth centuries, and originally written by an unlettered individual named John Howie, of Lochgoin. The book was greatly improved by the notes of Mr M’Gavin. He soon after published a refutation of the peculiar views of Mr Cobbett in his History of the Reformation; and a similar exposure of the principles of Mr Robert Owen. Being a decided enemy to the connexion of the church and state, he was induced to embody his sentiments on that subject in a pamphlet, entitled "Church Establishments considered; in a Series of Letters to a Covenantor." Not long before his death, Mr M’Gavin superintended a new and improved edition of Knox’s History of the Reformation; and aided with an introduction, a work by the Rev. Mr John Brown of Whitburn, entitled "Memorials of the Nonconformist Ministers of the Seventeenth Century." In the midst of his divers labours, he suddenly died of apoplexy, August 23, 1832.