John Keats was born in Moorgate, London, on 31 October 1795 to Thomas Keats and his wife, born Frances Jennings. There is little evidence of his exact birth place. Although Keats and his family seem to have marked his birthday on 29 October, baptism records give the date as the 31st. He was the eldest of four surviving children; his younger siblings were George (1797–1841), Thomas (1799–1818), and Frances Mary "Fanny" (1803–1889) who eventually married Spanish author Valentín Llanos Gutiérrez. Another son was lost in infancy. His father first worked as a hostler at the stables attached to the Swan and Hoop Inn, an establishment he later managed, and where the growing family lived for some years. Keats believed that he was born at the inn, a birthplace of humble origins, but there is no evidence to support his belief. The Globe pub now occupies the site (2012), a few yards from the modern-day Moorgate station. He was baptised at St Botolph-without-Bishopsgate, and sent to a local dame school as a child.

Letters and drafts of poems suggest that Keats first met Frances (Fanny) Brawne between September and November 1818. It is likely that the 18-year-old Brawne visited the Dilke family at Wentworth Place before she lived there. She was born in the hamlet of West End (now in the district of West Hampstead), on 9 August 1800. Like Keats's grandfather, her grandfather kept a London inn, and both lost several family members to tuberculosis. She shared her first name with both Keats's sister and mother, and had a talent for dress-making and languages as well as a natural theatrical bent. During November 1818 she developed an intimacy with Keats, but it was shadowed by the illness of Tom Keats, whom John was nursing through this period.

His parents were unable to afford Eton or Harrow, so in the summer of 1803, he was sent to board at John Clarke's school in Enfield, close to his grandparents' house. The small school had a liberal outlook and a progressive curriculum more modern than the larger, more prestigious schools. In the family atmosphere at Clarke's, Keats developed an interest in classics and history, which would stay with him throughout his short life. The headmaster's son, Charles Cowden Clarke, also became an important mentor and friend, introducing Keats to Renaissance literature, including Tasso, Spenser, and Chapman's translations. The young Keats was described by his friend Edward Holmes as a volatile character, "always in extremes", given to indolence and fighting. However, at 13 he began focusing his Energy on reading and study, winning his first academic prize in midsummer 1809.

In April 1804, when Keats was eight, his father died from a skull fracture, suffered when he fell from his horse while returning from a visit to Keats and his brother George at school. Thomas Keats died intestate. Frances remarried two months later, but left her new husband soon afterwards, and the four children went to live with their grandmother, Alice Jennings, in the village of Edmonton.

In March 1810, when Keats was 14, his mother died of tuberculosis, leaving the children in the custody of their grandmother. She appointed two guardians, Richard Abbey and John Sandell, to take care of them. That autumn, Keats left Clarke's school to apprentice with Thomas Hammond, a surgeon and apothecary who was a neighbour and the Doctor of the Jennings family. Keats lodged in the attic above the surgery at 7 Church Street until 1813. Cowden Clarke, who remained a close friend of Keats, described this period as "the most placid time in Keats's life."

When Keats died at 25, he had been writing poetry seriously for only about six years, from 1814 until the summer of 1820; and publishing for only four. In his lifetime, sales of Keats's three volumes of poetry probably amounted to only 200 copies. His first poem, the sonnet O Solitude appeared in the Examiner in May 1816, while his collection Lamia, Isabella, The Eve of St. Agnes and other poems was published in July 1820 before his last visit to Rome. The compression of his poetic apprenticeship and maturity into so short a time is just one remarkable aspect of Keats's work.

Having finished his apprenticeship with Hammond, Keats registered as a medical student at Guy's Hospital (now part of King's College London) and began studying there in October 1815. Within a month of starting, he was accepted as a dresser at the hospital, assisting Surgeons during operations, the equivalent of a junior house surgeon today. It was a significant promotion, that marked a distinct aptitude for medicine; it brought greater responsibility and a heavier workload. Keats's long and expensive medical training with Hammond and at Guy's Hospital led his family to assume he would pursue a lifelong career in Medicine, assuring financial security, and it seems that at this point Keats had a genuine Desire to become a Doctor. He lodged near the hospital, at 28 St Thomas's Street in Southwark, with other medical students, including Henry Stephens who became a famous Inventor and ink magnate.

In October 1816, Clarke introduced Keats to the influential Leigh Hunt, a close friend of Byron and Shelley. Five months later came the publication of Poems, the first volume of Keats's verse, which included "I stood tiptoe" and "Sleep and Poetry," both strongly influenced by Hunt. The book was a critical failure, arousing little interest, although Reynolds reviewed it favourably in The Champion. Clarke commented that the book "might have emerged in Timbuctoo." Keats's publishers, Charles and James Ollier, felt ashamed of the book. Keats immediately changed publishers to Taylor and Hessey on Fleet Street. Unlike the Olliers, Keats's new publishers were enthusiastic about his work. Within a month of the publication of Poems they were planning a new Keats volume and had paid him an advance. Hessey became a steady friend to Keats and made the company's rooms available for young Writers to meet. Their publishing lists eventually included Coleridge, Hazlitt, Clare, Hogg, Carlyle and Lamb.

Few of Keats's letters are extant from the period before he joined his literary circle. From spring 1817, however, there is a rich record of his prolific and impressive skills as letter Writer. Keats and his friends, poets, critics, novelists, and editors wrote to each other daily, and Keats' ideas are bound up in the ordinary, his day-to-day missives sharing news, parody and social commentary. They glitter with humour and critical intelligence. Born of an "unself-conscious stream of consciousness," they are impulsive, full of awareness of his own nature and his weak spots. When his brother George went to America, Keats wrote to him in great detail, the body of letters becoming "the real diary" and self-revelation of Keats's life, as well as containing an exposition of his philosophy, and the first drafts of poems containing some of Keats's finest writing and thought. Gittings describes them as akin to a "spiritual journal" not written for a specific other, so much as for synthesis.

"Ode on a Grecian Urn" and "Ode on Melancholy" were inspired by sonnet forms and probably written after "Ode to a Nightingale". Keats's new and progressive publishers Taylor and Hessey issued Endymion, which Keats dedicated to Thomas Chatterton, a work that he termed "a trial of my Powers of Imagination". It was damned by the critics, giving rise to Byron's quip that Keats was ultimately "snuffed out by an article", suggesting that he never truly got over it. A particularly harsh review by John Wilson Croker appeared in the April 1818 edition of The Quarterly Review. John Gibson Lockhart writing in Blackwood's Magazine, described Endymion as "imperturbable drivelling idiocy". With biting sarcasm, Lockhart advised, "It is a better and a wiser thing to be a starved apothecary than a starved poet; so back to the shop Mr John, back to plasters, pills, and ointment boxes". It was Lockhart at Blackwoods who coined the defamatory term "the Cockney School" for Hunt and his circle, which included both Hazlitt and Keats. The dismissal was as much political as literary, aimed at upstart young Writers deemed uncouth for their lack of education, non-formal rhyming and "low diction". They had not attended Eton, Harrow or Oxbridge and they were not from the upper classes.

Keats also reflected on the background and composition of his poetry, and specific letters often coincide with or anticipate the poems they describe. In February to May 1819 he produced many of his finest letters". Writing to his brother George, Keats explored the idea of the world as "the vale of Soul-making", anticipating the great odes that he would write some months later. In the letters, Keats coined ideas such as the Mansion of Many Apartments and the Chameleon Poet, concepts that came to gain Common currency and capture the public imagination, despite only making single appearances as phrases in his correspondence. The poetical mind, Keats argued:

Although prolific during his short career, and now one of the most studied and admired British poets, his reputation rests on a small body of work, centred on the Odes, and only in the creative outpouring of the last years of his short life was he able to express the inner intensity for which he has been lauded since his death. Keats was convinced that he had made no mark in his lifetime. Aware that he was dying, he wrote to Fanny Brawne in February 1820, "I have left no immortal work behind me – nothing to make my friends proud of my memory – but I have lov'd the principle of beauty in all things, and if I had had time I would have made myself remember'd. "

John Keats died in Rome on 23 February 1821 and was buried in the Protestant Cemetery, Rome. His last request was to be placed under a tombstone bearing no name or date, only the words, "Here lies One whose Name was writ in Water." Severn and Brown erected the stone, which under a relief of a lyre with broken strings, includes the epitaph:

None of Keats' biographies were written by people who had known him. Shortly after his death, his publishers announced they would speedily publish The memoirs and remains of John Keats but his friends refused to cooperate and argued with each other to the extent that the project was abandoned. Leigh Hunt's Lord Byron and some of his Contemporaries (1828) gives the first biographical account, strongly emphasising Keats's supposedly humble origins, a misconception which still continues. Given that he was becoming a significant figure within artistic circles, a succession of other publications followed, including anthologies of his many notes, chapters and letters. However, early accounts often gave contradictory or heavily biased versions of events and were subject to dispute. His friends Brown, Severn, Dilke, Shelley and his guardian Richard Abbey, his publisher Taylor, Fanny Brawne and many others issued posthumous commentary on Keats's life. These early writings coloured all subsequent biography and have become embedded in a body of Keats legend.

It took a month for the news of his death to reach London, after which Brawne stayed in mourning for six years. In 1833, more than 12 years after his death, she married and went on to have three children; she outlived Keats by more than 40 years.

Keats' letters were first published in 1848 and 1878. During the 19th century, critics deemed them unworthy of attention, distractions from his poetic works. During the 20th century they became almost as admired and studied as his poetry, and are highly regarded within the canon of English literary correspondence. T. S. Eliot described them as "certainly the most notable and most important ever written by any English poet." Keats spent a great deal of time considering poetry itself, its constructs and impacts, displaying a deep interest unusual amongst his milieu who were more easily distracted by metaphysics or politics, fashions or science. Eliot wrote of Keats's conclusions; "There is hardly one statement of Keats' about poetry which ... will not be found to be true, and what is more, true for greater and more mature poetry than anything Keats ever wrote."

There are areas of his life and daily routine that Keats does not describe. He mentions little about his childhood or his financial straits and is seemingly embarrassed to discuss them. There is a total absence of any reference to his parents. In his last year, as his health deteriorated, his concerns often gave way to despair and morbid obsessions. The publications of letters to Fanny Brawne in 1870 focused on this period and emphasised this tragic aspect, giving rise to widespread criticism at the time.

In 1882, Swinburne wrote in the Encyclopædia Britannica that "the Ode to a Nightingale, [is] one of the final masterpieces of human work in all time and for all ages". In the twentieth century, Keats remained the muse of poets such as Wilfred Owen, who kept his death date as a day of mourning, Yeats and T. S. Eliot. Critic Helen Vendler stated the odes "are a group of works in which the English language find ultimate embodiment". Bate declared of To Autumn: "Each generation has found it one of the most nearly perfect poems in English" and M. R. Ridley claimed the ode "is the most serenely flawless poem in our language."

John Keats: His Life and Death, the first major motion picture about the life of Keats, was produced in 1973 by Encyclopædia Britannica, Inc.. It was directed by John Barnes. John Stride played John Keats and Janina Faye played Fanny Brawne.

The largest collection of the letters, manuscripts, and other papers of Keats is in the Houghton Library at Harvard University. Other collections of material are archived at the British Library, Keats House, Hampstead, the Keats-Shelley Memorial House in Rome and the Pierpont Morgan Library in New York. Since 1998 the British Keats-Shelley Memorial Association have annually awarded a prize for romantic poetry. A Royal Society of Arts blue plaque was unveiled in 1896 to commemorate Keats at Keats House.

The 2009 film Bright Star, written and directed by Jane Campion, focuses on Keats' relationship with Fanny Brawne. Inspired by the 1997 Keats biography penned by Andrew Motion, it stars Ben Whishaw as Keats and Abbie Cornish as Fanny.

He used the term negative capability to discuss the state in which we are "capable of being in uncertainties, Mysteries, doubts without any irritable reaching after fact & reason ...[Being] content with half knowledge" where one trusts in the heart's perceptions. He wrote later: "I am certain of nothing but the holiness of the Heart's affections and the truth of Imagination – What the imagination seizes as Beauty must be truth – whether it existed before or not – for I have the same Idea of all our Passions as of Love they are all in their sublime, creative of essential Beauty" again and again turning to the question of what it means to be a poet. "My Imagination is a Monastery and I am its Monk", Keats notes to Shelley. In September 1819, Keats wrote to Reynolds "How beautiful the season is now – How fine the air. A temperate sharpness about it  ... I never lik'd the stubbled fields as much as now – Aye, better than the chilly green of spring. Somehow the stubble plain looks warm – in the same way as some pictures look warm – this struck me so much in my Sunday's walk that I composed upon it". The final stanza of his last great ode: "To Autumn" runs:

Although his poems were not generally well received by critics during his lifetime, his reputation grew after his death, and by the end of the 19th century, he had become one of the most beloved of all English poets. He had a significant influence on a diverse range of poets and Writers. Jorge Luis Borges stated that his first encounter with Keats's work was the most significant literary experience of his life.