He was educated at Harrow School and Caius College, Cambridge, where he took his B.A. degree in 1679. In the next year he was fined and dismissed from his college for having wounded a fellow-student with a sword. He was, however, received at Trinity Hall, where he took his M.A. degree in 1683.
After travelling in France and Italy, he settled in London, where he became acquainted with Dryden, Wycherley and others; and being made temporarily independent by inheriting a small fortune, he devoted himself to literature. The Duke of Marlborough procured him a place as one of the queen's waiters in the customs with a salary of £20 a year. This he afterwards disposed of for a small sum, retaining, at the suggestion of Lord Halifax, a yearly charge upon it for a long term of years.
Neither the poems nor the plays of Dennis are of any account, although one of his tragedies, a violent attack on the French in harmony with popular prejudice, entitled Liberty Asserted, was produced with great success at Lincoln's Inn Fields in 1704. His sense of his own importance approached mania, and he is said to have desired the Duke of Marlborough to have a special clause inserted in the Treaty of Utrecht to secure him from French vengeance. Marlborough pointed out that although he had been a still greater enemy of the French nation, he had no fear for his own security. This tale and others of a similar nature may well be exaggerations prompted by his enemies, but the infirmities of character and temper indicated in them were real.
Dennis is best remembered as a critic, and his earliest works, which lack the rancour that afterwards gained him the nickname of "Furius," are regarded as his best. They are:
Dennis had been offended by a humorous quotation made from his works by Addison, and published in 1713 Remarks upon Cato. Much of this criticism was acute and sensible, and it is quoted at considerable length by Johnson in his Life of Addison, but there is no doubt that Dennis was actuated by personal jealousy of Addison's success. Pope replied in The Narrative of Dr. Robert Norris, concerning the strange and deplorable frenzy of John Dennis ... (1713). This pamphlet was full of personal abuse, exposing Dennis's foibles, but offering no defence of Cato. Addison repudiated any connivance in this attack, and indirectly notified Dennis that when he did answer his objections, it would be without personalities.
Pope had already assailed Dennis in 1711 in the Essay on Criticism, as Appius. Dennis retorted by Reflections, Critical and Satirical ... a scurrilous production in which he taunted Pope with his deformity, saying among other things that he was as stupid and as venomous as a hunch-backed toad. He also wrote in 1715 Remarks upon Mr Pope's Translation of Homer ... and A True Character of Mr Pope. He accordingly figures in the Dunciad, and in a scathing note in the edition of 1729 (Book I, 1, 106) Pope quotes his more outrageous attacks, and adds an insulting epigram attributed to Richard Savage, but now generally ascribed to Pope.
More pamphlets followed, but Dennis's day was over. He outlived his annuity from the customs, and his last years were spent in great poverty. Bishop Atterbury sent him money, and he received a small sum annually from Sir Robert Walpole. A benefit performance was organized at the Haymarket (December 18, 1733) on his behalf. Pope wrote for the occasion an ill-natured prologue which Cibber recited. Dennis died within three weeks of this performance, on the 6th of January 1734.
His other works include several plays, for one of which, Appius and Virginia (1709), he invented a new kind of thunder. The play was not a success and the management of the Drury Lane Theatre withdrew it. But later at a performance of Macbeth there Dennis found the thunder produced by his method and said,
According to Brewer's entry (under the headword thunder), this is the origin of the phrase, "to steal one's thunder".
Dennis was one of the first writers to bring to prominence the concept of the sublime as an aesthetic quality. After taking the Grand Tour of the Alps he published his comments in a journal letter published as Miscellanies in 1693, giving an account of crossing the Alps where, contrary to his prior feelings for the beauty of nature as a "delight that is consistent with reason", the experience of the journey was at once a "pleasure to the eye as music is to the ear", but "mingled with Horrours, and sometimes almost with despair." The significance of his account is that the concept of the sublime, at the time a rhetoric term primarily relevant to literary criticism, was used to describe appreciation for horror and harmony when visually experiencing nature.
He wrote a curious Essay on the Operas after the Italian Manner (1706), maintaining that opera was the outgrowth of effeminate manners, and should, as such, be suppressed. His Works were published in 1702, Select Works (2 vol.) in 1718, and Miscellaneous Tracts, the first volume only of which appeared, in 1727.
For accounts of Dennis see Cibber's Lives of the Poets, Vol. 4; Isaac Disraeli's essays on Pope and Addison in the Quarrels of Authors, and On the Influence of a Bad Temper in Criticism in Calamities of Authors; and numerous references in Pope's Works.