He founded the Medical Society of London in 1773, convinced that a combined membership of physicians, surgeons and apothecaries would prove productive.

John and his brother were the sole survivors of seven sets of male twins, sons of Edward and Mary Lettsom.

Having completed an apprenticeship to a Yorkshire apothecary, Lettsom came to London in 1766 and through the influence of Dr Fothergill commenced his medical training at St Thomas' Hospital.

His studies were interrupted by the death of his father, prompting his return to Tortola in the British Virgin Islands, where he freed the slaves he had inherited and provided medical care for the local population.

As the only doctor in the islands at that time, he was able to earn a considerable sum, his diligence and industry enabling him to resume his studies in Europe.

Numerous other clubs, societies, hospitals, dispensaries, and charitable institutions in the United Kingdom and North America benefited from Lettsom's patronage, while from his pen there flowed a stream of "Hints", pamphlets, diatribes, and letters promoting Sunday schools, female industry, provision for the blind, a bee society, soup kitchens and the mangel-wurzel, while condemning quackery, card parties, and intemperance.

In 1791 Lettsom won the society's Fothergillian Prize for a treatise entitled Diseases of Great Towns and the Best Means of Preventing them.

Richard Woodman attributes similar words to the fictional naval surgeon Mr Lettsom in The Bomb Vessel:[16] When people's ill, they come to I, I physics, bleeds and sweats 'em; Sometime they live, sometimes they die, What's that to I?

Lettsom then set up a medical practice on Tortola, and as the only physician on the island amassed a small fortune of £2,000 in a mere six months, whereupon he gave half to his mother (who had remarried) and returned to London.

He wrote The naturalist's and traveller's companion, containing instructions for collecting and preserving objects of natural history and for promoting inquiries after human knowledge in general.