Born into an extremely wealthy family of Sussex gentry and heir to a baronetcy, Shelley received an education at Eton College and then went to the University of Oxford (University College). His first publication was a Gothic novel, Zastrozzi (1810), in which he gave vent to his atheistic worldview through the villain Zastrozzi. In the same year, Shelley together with his sister Elizabeth published Original Poetry by Victor and Cazire. After going up to Oxford, he issued a collection of (ostensibly burlesque but actually subversive) verse, Posthumous Fragments of Margaret Nicholson. A fellow-collegian, Thomas Jefferson Hogg, may have been his collaborator.
In 1811, Shelley published a pamphlet, The Necessity of Atheism, which resulted in his being made to leave Oxford, along with Hogg. He could have been reinstated, following the intervention of his father, had he recanted his avowed views. Shelley refused, which led to a total break between himself and his father.
In the same year, Shelley eloped to Scotland and married Harriet Westbrook, the daughter of a coffee-house keeper. Once married, Shelley moved to the Lake District to write, but shortly afterwards visited Ireland in order to engage in political pamphleteering. Two years later he published Queen Mab: A Philosophical Poem. The poem shows the influence of the British philosopher William Godwin, and much of Godwin's freethinking radical philosophy is voiced in it. By now unhappy in his marriage, Shelley fell in love with Godwin's and Mary Wollstonecraft's daughter, Mary. In July 1814 they eloped to Europe, crossing France and entering Switzerland. After six weeks, and out of money, they returned to England. The Shelleys would later publish an account of this voyage.
In the fall of 1815, while living close to London, Shelley produced the verse allegory Alastor, or The Spirit of Solitude. It attracted little attention at the time, but has come to be recognized as his first major poem.
In the summer of 1816 the Shelleys made a second trip to Switzerland. They were prompted to do so by Mary Shelley's half-sister Claire Clairmont, who had contracted a liaison with Lord Byron the previous April, just before he entered his self-exile on the continent. Byron had lost interest in Claire, and she used the opportunity of meeting the Shelleys as bait to lure him to Geneva. The Shelleys and Byron rented neighboring houses on the shores of Lake Geneva. Regular conversation with Byron had an invigorating effect on Shelley's poetry. A boating tour which the two took together inspired Shelley to write the Hymn to Intellectual Beauty, his first significant production since Alastor. A tour of Chamonix in the French Alps inspired Mont Blanc, a difficult poem in which Shelley ponders questions of historical inevitability and the relationship between the human mind and external nature. Shelley, in turn, influenced Byron's poetry. This new influence shows itself in the third part of Childe Harold's Pilgrimage, which Byron was working on, and in Manfred, which he wrote that fall. At the same time, Mary had been inspired to begin writing Frankenstein. At the end of summer, the Shelleys and Claire returned to England. Claire was pregnant with Byron's child, a fact that would have an enormous impact on Shelley's future.
The return to England was marred by tragedy. Fanny Kemble, a member of Godwin's household, killed herself in the late Autumn. In December 1816 Harriet Shelley committed suicide. A few weeks after her body was recovered from the Serpentine River in London's Hyde Park, Shelley and Mary Godwin were married. The marriage was intended, in part, to help secure Shelley's custody of his children by Harriet, but it was in vain: the children were given over to foster parents by the courts.
The Shelleys took up residence in the village of Marlow, where Shelley's friend Thomas Love Peacock lived. Shelley took part in the literary circle that surrounded Leigh Hunt, and during this period came to know John Keats. Shelley's major production this year was Laon and Cythna, a long narrative poem in which the two principal figures were incestuous lovers and which attacked religion. It was hastily withdrawn after only a few copies were published, then edited and reissued as The Revolt of Islam in 1818. Shelley also wrote two revolutionary political tracts under the nom de plume of "The Hermit of Marlow."
Early in 1818, the Shelleys and Claire left England in order to deliver the daughter of Byron and Claire to Byron, who had taken up residence in Venice. Again contact with Byron encouraged the production of Shelley's poetry. In the latter part of the year he wrote Julian and Maddalo and Prometheus Unbound. Tragedy struck in 1818 and 1819, when his infant daughter and son died of climate-related illnesses.
The Shelleys moved around various Italian cities during these years. Shelley completed Prometheus Unbound in Rome, and spent the summer of 1819 writing a tragedy, The Cenci, in Leghorn. In this year, propmpted among other causes by the Peterloo massacre, he wrote his best-known political poems, The Masque of Anarchy and Men of England, probably his best-remembered works during the 19th century, and the essay The Philosophical View of Reform, his most thorough exposition of his political views
In 1821, inspired by the death of John Keats, Shelley wrote the elegy Adonais.
In 1822 Shelley arranged for Leigh Hunt, the British poet and editor who had been one of his chief supporters in England, to come to Italy with his family; he intended that the three of them—himself, Byron and Hunt—would create a journal, to be called The Liberal, with Hunt as editor, which would disseminate their controversial writings and act as a counter-blast to conservative periodicals such as Blackwood's Magazine and The Quarterly Review.
On July 8, 1822, Shelley drowned in a sudden storm off Leghorn in the Bay of Spezia, while sailing back from Pisa and Leghorn to Lerici in his schooner, the Don Juan. He was returning from having set up The Liberal with the newly-arrived Hunt. The name "Don Juan", a compliment to Byron, was chosen by Edward Trelawny, a member of the Shelley-Byron Pisan circle, but according to Mary Shelley's testimony, Shelley changed it to "Ariel". This annoyed Byron, who caused "Don Juan" to be painted on the mainsail, giving offence to the Shelleys, who felt that the boat now looked like a coal-barge. The vessel, an open boat designed from a Royal dockyards model, was custom built in Genoa for Shelley. It did not capsize but sank; Mary Shelley declared in her "Note on Poems of 1822" (1839) that this design had a defect and was never seaworthy.
Shelley's body was washed ashore and later cremated on the beach near Viareggio. His heart was snatched, unconsumed, from the funeral pyre by Edward Trelawny, and kept by Mary Shelley until her dying day, while his ashes were interred in the Protestant Cemetery, Rome.
Three children survived him: Ianthe and Charles, his daughter and son by Harriet and Percy Florence, his son by Mary. Charles died of tuberculosis in 1826; Percy Florence, who eventually inherited the baronetcy in 1844, died without children. The only lineal descendants of the poet are therefore the children of Ianthe.
Unlike Byron, who despite his radical views had a large among the upper classes even while still alive, for decades after his death Shelley was read mainly among socialists and in the labor movement (Karl Marx was among his admirers). Only towards the end of the 19th century did his work, or rather his more innocuous work, become respectable.