Fothergill's success in his profession was rapid and assured, especially after the publication of his ‘Account of the Sore Throat,’ which greatly advanced his reputation, and before many years he had one of the largest and most lucrative practices in the city. But outside professional pursuits he took a keen and persistent interest in science and philanthropy, and holding no public appointments was able to give to these objects all his spare time. His chief scientific interest was in botany, especially in the collection and cultivation of rare plants. For this purpose he acquired an estate at Upton, near Stratford, where he laid out and kept up a magnificent botanical garden. In the words of an unquestionable authority, Sir Joseph Banks, ‘at an expense seldom undertaken by an individual, Dr. Fothergill procured from all parts of the world a great number of the rarest plants, and protected them in the amplest buildings which this or any other country has seen.’ He liberally paid those who brought plants which might be ornamental or useful to this country or her colonies. In richness his collection was, in Banks's opinion, equalled only by that in the royal gardens at Kew, while no other garden in Europe, even royal, had nearly so many scarce and valuable plants. To preserve a permanent record of these rarities, Fothergill kept several artists at work making figures of the new species. A list of the plants growing under glass was afterwards published by Dr. Lettsom, with the title ‘Hortus Uptonensis’ (Works, vol. iii.). But Fothergill's zeal was not merely the acquisitiveness of the collector. He was among the first to see the advantage of exchanging the vegetable products of different countries, and spent much energy and money in attempting to naturalise such plants as coffee, tea, and bamboo in America. His collections of shells and insects were also large and valuable; they mostly passed into the museum of Dr. William Hunter. A series of twelve hundred natural history drawings, done by the best artists, was bought after his death for a large sum by the empress of Russia.
Fothergill's philanthropic efforts were partly connected with the public benevolence of the Society of Friends. He took an active part in the foundation of the school for quaker children at Ackworth, to which he liberally contributed; he was interested in the funds raised for the relief of Spanish prisoners, and in numerous plans for improving the health, cleanliness, and prosperity of the working classes. But his private benevolence was also unceasing, and in some instances, such as that of Dr. Knight, librarian to the British Museum, whom he cleared from some embarrassments by a present of a thousand guineas, it was munificent. He assisted the production of important scientific works, such as those of Drury and Edwards, and he incurred the whole expense of printing a new translation of the Bible by Anthony Purver, a quaker. Fothergill took no part in current politics; but when troubles began to arise between England and the North American colonies, he made patriotic efforts to produce a better state of feeling. Having family connections with America and numerous correspondents there, he was better able than most persons to foresee the disastrous consequences of a mistaken policy, and in 1765 he wrote a pamphlet entitled ‘Considerations relative to the North American Colonies,’ in which he advocated the repeal of the Stamp Act. Even as late as 1774 he co-operated with Benjamin Franklin in drawing up a scheme of reconciliation, designed to be submitted to important persons on both sides, but perhaps never seriously considered by those in power.
The only weakness which was recognised in Fothergill's character, a certain obstinacy, may be credited with having led to his painful quarrel with Dr. Leeds. Fothergill was thought to have spoken ill of Leeds, who was also a quaker, and the matter being referred to arbitration, heavy damages were awarded to the latter. Fothergill refused to pay, and appealed to the court of king's bench. The court supported him, and the decision of a meeting of the Society of Friends was given in his favour (An Appeal to the People called Quakers on the Difference between S. Fothergill and S. Leeds, London, 1773). Fothergill's abstemious and regular habits assured him many years of good health. But in 1778 he began to suffer from a urinary disorder, which terminated his life on 26 Dec. 1780, and he was buried in the Friends' cemetery at Winchmore Hill 5 Dec. 1781. He was not married. His portrait by Hogarth is at the College of Physicians, and a head by R. Livesey, engraved by Bartolozzi, appears in the ‘Works.’ A bust and a medallion modelled by Flaxman were reproduced in Wedgwood ware. A life-sized bust was also taken of him in earlier life.
Fothergill's writings consisted chiefly of memoirs in the transactions of societies and a few separate tracts. They were all collected and reprinted in his ‘Works,’ edited by J. C. Lettsom, three vols. 4to and 8vo, 1783–4; also translated into German (Altenburg, 1785, two vols.). The most important is the ‘Account of the Sore Throat attended with Ulcers’ (first edit. 1748, sixth edit. 1777), which was translated into several European languages. It describes an epidemic of malignant sore throat or diphtheria which
