Shelley was born on 4 August 1792 at Field Place, Broadbridge Heath, near Horsham, West Sussex, England. He was the eldest son of Sir Timothy Shelley (1753–1844), a Whig Member of Parliament for Horsham from 1790 to 1792 and for Shoreham between 1806 and 1812, and his wife, Elizabeth Pilfold (1763–1846), the daughter of a successful butcher. He had four younger sisters and one much younger brother. Shelley’s early childhood was sheltered and mostly happy. He was particularly close to his sisters and his mother, who encouraged him to hunt, fish and ride.At age six, he was sent to a day school run by the vicar of Warnham church, where he displayed an impressive memory and gift for languages.

In 1802 he entered the Syon House Academy of Brentford, Middlesex.where his cousin Thomas Medwin was a pupil. Shelley was bullied and unhappy at the school and sometimes responded with violent rage. He also began suffering from the nightmares, hallucinations and sleep walking that were to periodically afflict him throughout his life. Shelley developed an interest in science which supplemented his voracious reading of tales of mystery, romance and the supernatural. During his holidays at Field Place, his sisters were often terrified at being subjected to his experiments with gunpowder, acids and electricity. Back at school he blew up a paling fence with gunpowder.

In 1804, Shelley entered Eton College, a period which he later recalled with loathing. He was subjected to particularly severe mob bullying called by the perpetrators “Shelley-baits”. A number of biographers and contemporaries have attributed the bullying to Shelley’s aloofness, nonconformity and refusal to take part in fagging. His peculiarities and violent rages earned him the nickname “Mad Shelley”. His interest in the occult and science continued, and contemporaries describe him giving an electric shock to a master, blowing up a tree stump with gunpowder and attempting to raise spirits with occult rituals. In his senior years, Shelley came under the influence of a part-time teacher, Dr James Lind, who encouraged his interest in the occult and introduced him to liberal and radical authors. According to Richard Holmes, by his leaving year, Shelley had gained a reputation as a classical scholar and a tolerated eccentric. In his last term, his first novel Zastrozzi appeared and he had established a following among his fellow students.

Prior to enrolling for University College, Oxford in October 1810, Shelley completed Original Poetry by Victor and Cazire (written with his sister Elizabeth), the verse melodrama The Wandering Jew and the gothic novel St. Irvine; or, The Rosicrucian: A Romance (published 1811).

In the winter of 1810-11, Shelley published a series of anonymous political poems and tracts: Posthumous Fragments of Margaret Nicholson, The Necessity of Atheism (written in collaboration with Hogg) and A Poetical Essay on the Existing State of Things. Shelley mailed The Necessity of Atheism to all the bishops and heads of colleges at Oxford, and he was called to appear before the college’s fellows, including the Dean, George Rowley. His refusal, to college authorities, to answer questions regarding whether or not he authored the pamphlet resulted in his expulsion from Oxford on 25 March 1811, along with Hogg. A number of writers have speculated that the expulsion of Shelley and Hogg was politically motivated. Hearing of his son’s expulsion, Shelley’s father threatened to cut all contact with Shelley unless he agreed to return home and study under tutors appointed by him. Shelley’s refusal to do so led to a falling-out with his father.

Four months after being sent down from Oxford, on 28 August 1811, the 19-year-old Shelley eloped to Scotland with the 16-year-old Harriet Westbrook, a pupil at the same boarding school as Shelley’s sisters, whom his father had forbidden him to see. Harriet Westbrook had been writing Shelley passionate letters threatening to kill herself because of her unhappiness at the school and at home. Shelley, heartbroken after the failure of his romance with his cousin, Harriet Grove, cut off from his mother and sisters, and convinced he had not long to live, impulsively decided to rescue Westbrook and make her his beneficiary. Westbrook’s 28-year-old sister Eliza, to whom Harriet was very close, appears to have encouraged the young girl’s infatuation with the future baronet. The Westbrooks pretended to disapprove but secretly encouraged the elopement. Sir Timothy Shelley, however, outraged that his son had married beneath him (Harriet’s father, though prosperous, had kept a tavern), revoked Shelley’s allowance and refused ever to receive the couple at Field Place. Harriet also insisted that her sister Eliza, whom Shelley detested, live with them. Shelley invited his friend Hogg to share his ménage but asked him to leave when Hogg made advances to Harriet. Shelley was also at this time increasingly involved in an intense platonic relationship with Elizabeth Hitchener, a 28-year-old unmarried schoolteacher of advanced views, with whom he had been corresponding. Hitchener, whom Shelley called the “sister of my soul” and “my second self”, became his muse and confidante in the writing of his philosophical poem Queen Mab, a Utopian allegory.

Shelley wrote several essays on the subject of vegetarianism, the more prominent of which were “A Vindication of Natural Diet” (1813) and “On the Vegetable System of Diet”. Shelley’s eagerness for vegetarianism is connected with India. In 1812 he was converted to vegetarianism by his friend John Frank Newton, who had himself been converted while living in India.

On 28 July 1814 Shelley abandoned Harriet, now pregnant with their son Charles (November 1814 – 1826) and (in imitation of the hero of one of Godwin’s novels) he ran away to Switzerland with Mary, then 16, inviting her stepsister Claire Clairmont (also 16) along because she could speak French. The older sister Fanny was left behind, to her great dismay, for she, too, may have fallen in love with Shelley. The three sailed to Europe, and made their way across France to Switzerland on foot, reading aloud from the works of Rousseau, Shakespeare, and Mary’s mother, Mary Wollstonecraft (an account of their travels was subsequently published by the Shelleys).

After six weeks, homesick and destitute, the three young people returned to England. The enraged William Godwin refused to see them, though he still demanded money (to be given to him under another name, to avoid scandal). In late 1815, while living in a cottage in Bishopsgate, Surrey, with Mary and avoiding creditors, Shelley wrote Alastor, or The Spirit of Solitude. It attracted little attention at the time, but has now come to be recognised as his first major achievement. At this point in his writing career, Shelley was deeply influenced by the poetry of Wordsworth.

Shelley and Mary returned to England in September 1816, and in early October they heard that Mary’s half-sister Fanny Imlay had killed herself. Godwin believed that Fanny had been in love with Shelley, and Shelley himself suffered depression and guilt over her death, writing: “Friend had I known thy secret grief/Should we had parted so.” Further tragedy followed in December when Shelley’s estranged wife Harriet drowned herself in The Serpentine. Harriet, pregnant and living alone at the time, believed that she had been abandoned by her new lover. In her suicide letter she asked Shelley to take custody of their son Charles but to leave their daughter in her sister Eliza’s care.

In mid-1816 Shelley and Mary made a second trip to Switzerland. They were prompted to do this by Mary’s stepsister Claire Clairmont, who, in competition with Mary, had initiated a liaison with Lord Byron the previous April just before his self-exile on the continent. Byron’s interest in her had waned, and Claire used the opportunity of introducing him to Mary and Shelley to act as bait to lure him to Geneva. The couple and Byron rented neighbouring houses on the shores of Lake Geneva. Regular conversation with Byron had an invigorating effect on Shelley’s output of poetry. While on a boating tour the two took together, Shelley was inspired to write his Hymn to Intellectual Beauty, often considered his first significant production since Alastor. A tour of Chamonix in the French Alps inspired Mont Blanc, a poem in which Shelley claims to have pondered questions of historical inevitability (determinism) and the relationship between the human mind and external nature. Shelley also encouraged Byron to begin an epic poem on a contemporary subject, advice that resulted in Byron’s composition of Don Juan. In 1817 Claire gave birth to a daughter by Byron, Alba, later renamed Allegra, whom Shelley offered to support, making provisions for her and for Claire in his will.

In March 1817 the Shelleys moved to the village of Marlow, Buckinghamshire, where Shelley’s friend Thomas Love Peacock, lived. The Shelley household included Claire and her baby Allegra, both of whose presence was resented by Mary. Shelley’s generosity with money and increasing debts also led to financial and marital stress, as did Godwin’s frequent requests for financial help.

Shelley took part in the literary and political circle that surrounded Leigh Hunt, and during this period he met William Hazlitt and John Keats. Shelley’s major work during this time was Laon and Cythna, a long narrative poem featuring incest and attacks on religion. It was hastily withdrawn after publication due to fears of prosecution for religious libel, and was re-edited and reissued as The Revolt of Islam in January 1818. Shelley also published two political tracts under a pseudonym: A Proposal for putting Reform to the Vote throughout the Kingdom (March 1817) and An Address to the People on the Death of Princess Charlotte (November 1817) In December he wrote “Ozymandias” which is considered to be one of his finest sonnets.

On 12 March 1818 the Shelleys and Claire left England to escape its “tyranny civil and religious.” A doctor had also recommended that Shelley go to Italy for his chronic lung complaint, and Shelley had arranged to take Claire’s daughter, Allegra, to her father Byron who was now in Venice. Two days before they left, William, Clara and Allegra were baptised at the church of St Giles-in-the-Fields.

The Shelleys moved to Naples on 1 December where they stayed for three months. During this period Shelley was ill, depressed and almost suicidal: a state of mind reflected in his poem “Stanzas written in Dejection-December 1818, Near Naples”.

In Rome, Shelley was in poor health, probably suffering from nephritis and tuberculosis which later was in remission. Nevertheless, he made significant progress on three major works: Julian and Maddalo, Prometheus Unbound, and The Cenci. Julian and Maddalo is an autobiographical poem which explores the relationship between Shelley and Byron and analyses Shelley’s personal crises of 1818 and 1819. The poem was completed in the summer of 1819, but was not published in Shelley’s lifetime. Prometheus Unbound is a long dramatic poem inspired by Aeschylus’s retelling of the Prometheus myth. It was completed in late 1819 and published in 1820. The Cenci is a verse drama of rape, murder and incest based on the story of the Renaissance Count Cenci of Rome and his daughter Beatrice. Shelley completed the play in September and the first edition was published that year. It was to become one of his most popular works and the only one to have two authorised editions in his lifetime.

While in Naples, Shelley registered the birth and baptism of a baby girl, Elena Adelaide Shelley (born 27 December) and named himself as the father. The parentage of Elena has never been conclusively established, and she was to die in a poor suburb of Naples on 9 June 1820. Shelley registered the birth and baptism on 27 February 1820, and the household left Naples for Rome the following day, leaving Elena with carers.

The Shelleys moved to Pisa in January 1820, ostensibly to consult a doctor who had been recommended to them. In March Shelley wrote to friends that Mary was depressed, suicidal and hostile towards him. Shelley was also beset by financial worries, as creditors from England pressed him for payment and he was obliged to make secret payments in connection with his “Neopolitan charge” Elena.

In July, hearing that John Keats’s was seriously ill in England, Shelley wrote to the poet inviting him to stay with him at Pisa. Keats replied with hopes of seeing him, but instead, arrangements were made for Keats to travel to Rome. Following the death of Keats in 1821, Shelley wrote Adonais which Harold Bloom considers one of the major pastoral elegies. The poem was published in Pisa in July 1821, but sold few copies.

In March 1821 Shelley completed “A Defence of Poetry”, a response to Peacock’s article “The Four Ages of Poetry”. Shelley’s essay, with its famous conclusion “Poets are the unacknowledged legislators of the world”, remained unpublished in his lifetime.

Shelley’s ashes were reburied in a different plot at the cemetery in 1823. His grave bears the Latin inscription, Cor Cordium (Heart of Hearts), and a few lines of “Ariel’s Song” from Shakespeare’s The Tempest:

Three children survived Shelley: Ianthe and Charles, his daughter and son by Harriet; and Percy Florence Shelley, his son by Mary. Charles, who suffered from tuberculosis, died in 1826 after being struck by lightning during a rainstorm. Percy Florence, who eventually inherited the baronetcy in 1844, died without children “of his body”, as the old legal phrase went.

Shelley’s poem “To the Queen of My Heart” was allegedly forged and falsely attributed to Shelley by James Augustus St. John, who took over as editor of the London Weekly Review when Carlile was imprisoned in 1827.

The Shelleys were now living in Livorno where Shelley completed Julian and Maddalo, and The Cenci. In September Shelley heard of the “Peterloo” massacre of peaceful protesters in Manchester and within two weeks had completed one of his most famous political poems, The Mask of Anarchy, and despatched it to Leigh Hunt for publication. Hunt, however, decided not to publish because of a justifiable fear of prosecution for seditious libel. The poem was only officially published in 1832.

The only lineal descendants of the poet are therefore the children of Ianthe. Ianthe Eliza Shelley was married in 1837 to Edward Jeffries Esdaile of Cothelstone Manor, grandson of the banker William Esdaile of Lombard Street, London. The marriage resulted in the birth of three daughters, Ianthe Harriet Shelley (1839–1849), Eliza Margaret (1841–1930), and Mary Emily Sydney (1848–1854), and three sons, Charles Edward (1842–1842), Charles Edward Jeffries (1845–1922), and William (1846–1915). Ianthe died in 1876, and her only descendants result from the marriage of Charles Edward Jeffries Esdaile and Marion Maxwell Sandbach.

On 12 November, Mary gave birth to a boy, Percy Florence Shelley. Percy was to inherit the baronetcy in 1844 and died as Sir Percy Shelley in 1889.

Several members of the Scarlett family were born at Percy Florence’s seaside home “Boscombe Manor” in Bournemouth. They were descendants of Percy Florence’s and Jane Gibson’s adopted daughter, Bessie Florence Gibson. The 1891 census shows Jane Shelley, Percy Florence Shelley’s widow, living at Boscombe Manor with several great-nephews. Percy Florence Shelley died in 1889, and his widow, the former Jane St. John (born Gibson), died in 1899.

In 2005 the University of Delaware Press published an extensive two-volume biography by James Bieri. In 2008 the Johns Hopkins University Press published Bieri’s 856-page one-volume biography, Percy Bysshe Shelley: A Biography.

In 2007, John Lauritsen published The Man Who Wrote Frankenstein, making the controversial claim that Percy’s contributions to Mary Shelley’s Frankenstein were extensive. In 2008 Charles E. Robinson credited Percy as the co-author of Frankenstein in The Original Frankenstein published by the Bodleian Library in Oxford and by Random House in the US. More modern scholars strongly dispute these findings, including Professor Charlotte Gordon in Romantic Outlaws: The Extraordinary Lives of Mary Wollstonecraft and Mary Shelley, published in 2015. The celebrated poet and literary scholar Fiona Sampson said, on the 200th anniversary of Frankenstein’s creation, “In recent years Percy’s corrections, visible in the Frankenstein notebooks held at the Bodleian Library in Oxford, have been seized on as evidence that he must have at least co-authored the novel. In fact, when I examined the notebooks myself, I realised that Percy did rather less than any line editor working in publishing today.”

In late 2014 Shelley’s work led lecturers from the University of Pennsylvania and New York University to produce a massive open online course (MOOC) on the life of Percy Shelley and Prometheus Unbound.

The rediscovery in mid-2006 of Shelley’s long-lost “Poetical Essay on the Existing State of Things”, as noted above, was slow to be followed up until the only known surviving copy was acquired by the Bodleian Library in Oxford as its 12-millionth book in November 2015 and made available online. An analysis of the poem by the only person known to have examined the whole work at the time of the original discovery appeared in the Times Literary Supplement: H.R. Woudhuysen, “Shelley’s Fantastic Prank”, 12 July 2006.