He was the eldest son of Sir Roger Jenyns and his second wife Elizabeth Soame, the daughter of Sir Peter Soame. He was born in London, and was educated at St John's College, Cambridge. In 1742 he was chosen M.P. for Cambridgeshire, in which his property was situated, and he afterwards sat for the borough of Dunwich and the town of Cambridge. From 1755 to 1780 he was one of the commissioners of the Board of Trade. He was elected as a Bailiff to the board of the Bedford Level Corporation for 1748–69 and 1771–87. For the measure of literary repute which he enjoyed during his life Jenyns was indebted as much to his wealth and social standing as to his accomplishments and talents, though both were considerable. His poetical works, the Art of Dancing and Miscellanies, contain many passages graceful and lively though occasionally verging on licence. The first of his prose works was his Free Inquiry into the Nature and Origin of Evil. This essay was severely criticized on its appearance, especially by Samuel Johnson in the Literary Magazine. Johnson condemned the book as a slight and shallow attempt to solve one of the most difficult of moral problems. Jenyns, a gentle and amiable man in the main, was extremely irritated by his review. He put forth a second edition of his work, prefaced by a vindication, and tried to take vengeance on Johnson after his death by a sarcastic epitaph: In 1776 Jenyns published his View of the Internal Evidence of the Christian Religion. Though at one period of his life he had affected a kind of deistic scepticism, he had now returned to orthodoxy, and there seems no reason to doubt his sincerity, questioned at the time, in defending Christianity on the ground of its total agreement with the principles of human reason. The work was deservedly praised for its literary merits.
Marriages
He married twice but left no progeny:
Firstly to Mary Soame, only daughter of Col. Edmund Soame of Dereham, Norfolk, a Member of Parliament for Thetford in Norfolk from 1701 to 1705, who fought for King William III. His life-size alabaster statue survives in West Dereham Church.
Secondly he married Elizabeth Grey, daughter of Henry Grey of Hackney, Middlesex.
A collected edition of the works of Jenyns appeared in 1790, with a biography by Charles Nalson Cole. There are several references to him in James Boswell's Johnson.
Commentary on Jenyns
describes Jenyns' take on the American Revolution in The Eve of the Revolution as follows:
