John Milton was born in Bread Street, London on 9 December 1608, the son of Composer John Milton and his wife Sarah Jeffrey. The senior John Milton (1562–1647) moved to London around 1583 after being disinherited by his devout Catholic father Richard Milton for embracing Protestantism. In London, the senior John Milton married Sarah Jeffrey (1572–1637) and found lasting financial success as a scrivener. He lived in and worked from a house on Bread Street, where the Mermaid Tavern was located in Cheapside. The elder Milton was noted for his skill as a musical Composer, and this talent left his son with a lifelong appreciation for music and friendships with Musicians such as Henry Lawes.

Milton's views developed from his very extensive reading, as well as travel and experience, from his student days of the 1620s to the English Civil War. By the time of his death in 1674, Milton was impoverished and on the margins of English intellectual life, yet famous throughout Europe and unrepentant for his political choices.

Milton and his first wife Mary Powell (1625–1652) had four children:

Milton may have been rusticated (suspended) in his first year for quarrelling with his tutor, Bishop william Chappell. He was certainly at home in London in the Lent Term 1626; there he wrote his Elegia Prima, a first Latin elegy, to Charles Diodati, a friend from St Paul's. Based on remarks of John Aubrey, Chappell "whipt" Milton. This story is now disputed, though certainly Milton disliked Chappell. Historian Christopher Hill cautiously notes that Milton was "apparently" rusticated, and that the differences between Chappell and Milton may have been either religious or personal. It is also possible that, like Isaac Newton four decades later, Milton was sent home because of the plague, by which Cambridge was badly affected in 1625. In 1626, Milton's tutor was Nathaniel Tovey.

Milton's poetry was slow to see the light of day, at least under his name. His first published poem was "On Shakespeare" (1630), anonymously included in the Second Folio edition of william Shakespeare's plays in 1632. Milton collected his work in 1645 Poems in the midst of the excitement attending the possibility of establishing a new English government. The anonymous edition of Comus was published in 1637, and the publication of Lycidas in 1638 in Justa Edouardo King Naufrago was signed J. M. Otherwise. The 1645 collection was the only poetry of his to see print until Paradise Lost appeared in 1667.

Milton continued to write poetry during this period of study; his Arcades and Comus were both commissioned for masques composed for noble patrons, connections of the Egerton family, and performed in 1632 and 1634 respectively. Comus argues for the virtuousness of temperance and chastity. He contributed his pastoral elegy Lycidas to a memorial collection for one of his fellow-students at Cambridge. Drafts of these poems are preserved in Milton’s poetry notebook, known as the Trinity Manuscript because it is now kept at Trinity College, Cambridge.

He first went to Calais and then on to Paris, riding horseback, with a letter from diplomat Henry Wotton to ambassador John Scudamore. Through Scudamore, Milton met Hugo Grotius, a Dutch law Philosopher, Playwright, and poet. Milton left France soon after this meeting. He travelled south from Nice to Genoa, and then to Livorno and Pisa. He reached Florence in July 1638. While there, Milton enjoyed many of the sites and structures of the city. His candour of manner and erudite neo-Latin poetry earned him friends in Florentine intellectual circles, and he met the Astronomer Galileo who was under house arrest at Arcetri, as well as others. Milton probably visited the Florentine Academy and the Academia della Crusca along with smaller academies in the area, including the Apatisti and the Svogliati.

Originally, Milton wanted to leave Naples in order to travel to Sicily and then on to Greece, but he returned to England during the summer of 1639 because of what he claimed in Defensio Secunda were "sad tidings of civil war in England." Matters became more complicated when Milton received word that his childhood friend Diodati had died. Milton in fact stayed another seven months on the continent, and spent time at Geneva with Diodati's uncle after he returned to Rome. In Defensio Secunda, Milton proclaimed that he was warned against a return to Rome because of his frankness about religion, but he stayed in the city for two months and was able to experience Carnival and meet Lukas Holste, a Vatican librarian who guided Milton through its collection. He was introduced to Cardinal Francesco Barberini who invited Milton to an opera hosted by the Cardinal. Around March, Milton travelled once again to Florence, staying there for two months, attending further meetings of the academies, and spending time with friends. After leaving Florence, he travelled through Lucca, Bologna, and Ferrara before coming to Venice. In Venice, Milton was exposed to a model of Republicanism, later important in his political writings, but he soon found another model when he travelled to Geneva. From Switzerland, Milton travelled to Paris and then to Calais before finally arriving back in England in either July or August 1639.

In his 1641 treatise, Of Reformation, Milton expressed his dislike for Catholicism and episcopacy, presenting Rome as a modern Babylon, and bishops as Egyptian taskmasters. These analogies conform to Milton's puritanical preference for Old Testament imagery. He knew at least four commentaries on Genesis: those of John Calvin, Paulus Fagius, David Pareus and Andreus Rivetus.

In June 1642, Milton paid a visit to the manor house at Forest Hill, Oxfordshire, and returned with 16 year-old bride Mary Powell. Mary found life difficult with the severe 35 year-old schoolmaster and pamphleteer, and she returned to her family a month later. She did not return until 1645, partly because of the outbreak of the Civil War.

Milton wrote during a period when thoughts about divorce were anything but simplistic; rather, there was active debate among thinkers and intellectuals at the time. However, Milton's basic approval of divorce within strict parameters set by the biblical witness was typical of many influential Christian intellectuals, particularly the Westminster divines. Milton addressed the Assembly on the matter of divorce in August 1643, at a moment when the Assembly was beginning to form its opinion on the matter. In the Doctrine & Discipline of Divorce, Milton argued that divorce was a private matter, not a legal or ecclesiastical one. Neither the Assembly nor Parliament condemned Milton or his ideas. In fact, when the Westminster Assembly wrote the Westminster Confession of Faith they allowed for divorce ('Of Marriage and Divorce,' Chapter 24, Section 5) in cases of infidelity or abandonment. Thus, the Christian community, at least a majority within the 'Puritan' sub-set, approved of Milton's views.

He was supported by his father's Investments, but Milton became a private schoolmaster at this time, educating his nephews and other children of the well-to-do. This experience and discussions with educational reformer Samuel Hartlib led him to write his short tract Of Education in 1644, urging a reform of the national universities.

Even here, though, his originality is qualified: Thomas Gataker had already identified "mutual solace" as a principal goal in marriage. Milton abandoned his campaign to legitimise divorce after 1645, but he expressed support for polygamy in the De Doctrina Christiana, the theological treatise that provides the clearest evidence for his views.

A friend and ally in the pamphlet wars was Marchamont Nedham. Austin Woolrych considers that although they were quite close, there is "little real affinity, beyond a broad republicanism", between their approaches. Blair Worden remarks that both Milton and Nedham, with others such as Andrew Marvell and James Harrington, would have taken their Problem with the Rump Parliament to be not the republic itself, but the fact that it was not a proper republic. Woolrych speaks of "the gulf between Milton's vision of the Commonwealth's Future and the reality". In the early version of his History of Britain, begun in 1649, Milton was already writing off the members of the Long Parliament as incorrigible.

Milton embraced many heterodox Christian theological views. He rejected the Trinity, in the belief that the Son was subordinate to the Father, a position known as Arianism; and his sympathy or curiosity was probably engaged by Socinianism: in August 1650 he licensed for publication by william Dugard the Racovian Catechism, based on a non-trinitarian creed. A source has interpreted him as broadly Protestant, if not always easy to locate in a more precise religious category.

He praised Oliver Cromwell as the Protectorate was set up; though subsequently he had major reservations. When Cromwell seemed to be backsliding as a revolutionary, after a couple of years in power, Milton moved closer to the position of Sir Henry Vane, to whom he wrote a sonnet in 1652. The group of disaffected republicans included, besides Vane, John Bradshaw, John Hutchinson, Edmund Ludlow, Henry Marten, Robert Overton, Edward Sexby and John Streater; but not Marvell, who remained with Cromwell's party. Milton had already commended Overton, along with Edmund Whalley and Bulstrode Whitelocke, in Defensio Secunda. Nigel Smith writes that

By 1654, Milton had become totally blind; the cause of his blindness is debated but bilateral retinal detachment or glaucoma are most likely. His blindness forced him to dictate his verse and prose to amanuenses who copied them out for him; one of these was poet Andrew Marvell. One of his best-known sonnets is presumed to date from this period, When I Consider How My Light is Spent, titled by a later Editor "On His Blindness".

On 12 November 1656, Milton was married to Katherine Woodcock at St Margaret's, Westminster. She died on 3 February 1658, less than four months after giving birth to her daughter Katherine, who also died.

Milton's magnum opus, the blank-verse epic poem Paradise Lost, was composed by the blind and impoverished Milton from 1658 to 1664 (first edition), with small but significant revisions published in 1674 (second edition). As a blind poet, Milton dictated his verse to a series of aides in his employ. It has been argued that the poem reflects his personal despair at the failure of the Revolution, yet affirms an ultimate optimism in human potential. Some literary critics have argued that Milton encoded many references to his unyielding support for the "Good Old Cause".

The Restoration of the Stuart monarchy in 1660 began a new phase in Milton's work. In Paradise Lost, Paradise Regained and Samson Agonistes, Milton mourns the end of the godly Commonwealth. The Garden of Eden may allegorically reflect Milton's view of England's recent Fall from Grace, while Samson's blindness and captivity—mirroring Milton's own lost sight—may be a metaphor for England's blind acceptance of Charles II as king. Illustrated by Paradise Lost is mortalism, the belief that the soul lies dormant after the body dies.

Milton married for a third time on 24 February 1663 to Elizabeth Mynshull or Minshull (1638–1728), the niece of Thomas Mynshull, a wealthy apothecary and philanthropist in Manchester. The marriage took place at St Mary Aldermary in the City of London. Despite a 31-year age gap, the marriage seemed happy, according to John Aubrey, and lasted more than 12 years until Milton's death. (A plaque on the wall of Mynshull's House in Manchester describes Elizabeth as Milton's "3rd and Best wife".) Samuel Johnson, however, claims that Mynshull was "a domestic companion and attendant" and that Milton's nephew Edward Phillips relates that Mynshull "oppressed his children in his lifetime, and cheated them at his death".

On 27 April 1667, Milton sold the publication rights for Paradise Lost to publisher Samuel Simmons for £5 (equivalent to approximately £770 in 2015 purchasing power), with a further £5 to be paid if and when each print run sold out of between 1,300 and 1,500 copies. The first run was a quarto edition priced at three shillings per copy (about £23 in 2015 purchasing power equivalent), published in August 1667, and it sold out in eighteen months.

Milton followed up the publication Paradise Lost with its sequel Paradise Regained, which was published alongside the tragedy Samson Agonistes in 1671. Both of these works also resonate with Milton's post-Restoration political situation. Just before his death in 1674, Milton supervised a second edition of Paradise Lost, accompanied by an explanation of "why the poem rhymes not", and prefatory verses by Andrew Marvell. In 1673, Milton republished his 1645 Poems, as well as a collection of his letters and the Latin prolusions from his Oxford days.

During this period, Milton published several minor prose works, such as the grammar textbook Art of Logic and a History of Britain. His only explicitly political tracts were the 1672 Of True Religion, arguing for toleration (except for Catholics), and a translation of a Polish tract advocating an elective monarchy. Both these works were referred to in the Exclusion debate, the attempt to exclude the heir presumptive from the throne of England—James, Duke of York—because he was Roman Catholic. That debate preoccupied politics in the 1670s and 1680s and precipitated the formation of the Whig party and the Glorious Revolution.

Milton died of kidney failure on 8 November 1674 and was buried in the church of St Giles Cripplegate, Fore Street, London. According to an early biographer, his funeral was attended by “his learned and great Friends in London, not without a friendly concourse of the Vulgar.” A monument was added in 1793, sculpted by John Bacon the Elder.

John Dryden, an early enthusiast, in 1677 began the trend of describing Milton as the poet of the sublime. Dryden's The State of Innocence and the Fall of Man: an Opera (1677) is evidence of an immediate cultural influence. In 1695, Patrick Hume became the first Editor of Paradise Lost, providing an extensive apparatus of annotation and commentary, particularly chasing down allusions.

In 1732, the classical scholar Richard Bentley offered a corrected version of Paradise Lost. Bentley was considered presumptuous, and was attacked in the following year by Zachary Pearce. Christopher Ricks judges that, as critic, Bentley was both acute and wrong-headed, and "incorrigibly eccentric"; william Empson also finds Pearce to be more sympathetic to Bentley's underlying line of thought than is warranted.

Milton's use of blank verse, in addition to his stylistic innovations (such as grandiloquence of voice and vision, peculiar diction and phraseology) influenced later poets. At the time, poetic blank verse was considered distinct from its use in verse drama, and Paradise Lost was taken as a unique examplar. Said Isaac Watts in 1734, "Mr. Milton is esteemed the parent and author of blank verse among us". "Miltonic verse" might be synonymous for a century with blank verse as poetry, a new poetic terrain independent from both the drama and the heroic couplet.

While neo-classical diction was as restrictive as its prosody, and narrow imagery paired with uniformity of sentence structure resulted in a small set of 800 nouns circumscribing the vocabulary of 90% of heroic couplets ever written up to the eighteenth century, and tradition required that the same adjectives attach to the same nouns, followed by the same verbs, Milton's pursuit of liberty extended into his vocabulary as well. It included many Latinate neologisms, as well as obsolete words already dropped from popular usage so completely that their meanings were no longer understood. In 1740, Francis Peck identified some examples of Milton's "old" words (now popular). The “Miltonian dialect” as it was called, was emulated by later poets; Pope used the diction of Paradise Lost in his Homer translation, while the lyric poetry of Gray and Collins was frequently criticised for their use of “obsolete words out of Spenser and Milton”. The language of Thomson's finest poems (e.g. The Seasons, Castle of Indolence) was self-consciously modelled after the Miltonian dialect, with the same tone and sensibilities as Paradise Lost. Following to Milton, English poetry from Pope to John Keats exhibited a steadily increasing attention to the connotative, the imaginative and poetic, value of words.

Many enlightenment thinkers of the 18th century revered and commented on Milton's poetry and non-poetical works. In addition to John Dryden, among them were Alexander Pope, Joseph Addison, Thomas Newton, and Samuel Johnson. For Example, in The Spectator, Joseph Addison wrote extensive notes, annotations, and interpretations of certain passages of Paradise Lost. Jonathan Richardson, senior, and Jonathan Richardson, the younger, co-wrote a book of criticism. In 1749, Thomas Newton published an extensive edition of Milton's poetical works with annotations provided by himself, Dryden, Pope, Addison, the Richardsons (father and son) and others. Newton's edition of Milton was a culmination of the honour bestowed upon Milton by early Enlightenment thinkers; it may also have been prompted by Richard Bentley's infamous edition, described above. Samuel Johnson wrote numerous essays on Paradise Lost, and Milton was included in his Lives of the Most Eminent English Poets (1779–1781).

William Hayley's 1796 biography called him the "greatest English author", and he remains generally regarded "as one of the preeminent Writers in the English language", though critical reception has oscillated in the centuries since his death (often on account of his republicanism). Samuel Johnson praised Paradise Lost as "a poem which...with respect to design may claim the first place, and with respect to performance, the second, among the productions of the human mind", though he (a Tory and recipient of royal patronage) described Milton's politics as those of an "acrimonious and surly republican".

The Romantic poets valued his exploration of blank verse, but for the most part rejected his religiosity. william Wordsworth began his sonnet "London, 1802" with "Milton! thou should'st be living at this hour" and modelled The Prelude, his own blank verse epic, on Paradise Lost. John Keats found the yoke of Milton's style uncongenial; he exclaimed that "Miltonic verse cannot be written but in an artful or rather artist's humour." Keats felt that Paradise Lost was a "beautiful and grand curiosity", but his own unfinished attempt at epic poetry, Hyperion, was unsatisfactory to the author because, amongst other things, it had too many "Miltonic inversions". In The Madwoman in the Attic, Sandra Gilbert and Susan Gubar note that Mary Shelley's novel Frankenstein is, in the view of many critics, "one of the key 'Romantic' readings of Paradise Lost."

An unfinished religious manifesto, De doctrina christiana, probably written by Milton, lays out many of his heterodox theological views, and was not discovered and published until 1823. Milton's key beliefs were idiosyncratic, not those of an identifiable group or faction, and often they go well beyond the orthodoxy of the time. Their tone, however, stemmed from the Puritan emphasis on the centrality and inviolability of conscience. He was his own man, but he was anticipated by Henry Robinson in Areopagitica.

Milton's ode At a solemn Musick was set for choir and orchestra as Blest Pair of Sirens by Hubert Parry (1848–1918), and Milton's poem On the Morning of Christ's Nativity was set as a large-scale choral work by Cyril Rootham (1875–1938). Milton also wrote the hymn Let us with a gladsome mind, a versification of Psalm 136. His 'L'Allegro' and 'Il Penseroso', with additional material, were magnificently set by Handel (1740).

Milton's Areopagitica is still cited as relevant to the First Amendment to the United States Constitution. A quotation from Areopagitica—"A good book is the precious lifeblood of a master spirit, embalmed and treasured up on purpose to a life beyond life"—is displayed in many public libraries, including the New York Public Library.

Once Paradise Lost was published, Milton's stature as epic poet was immediately recognised. He cast a formidable Shadow over English poetry in the 18th and 19th centuries; he was often judged equal or superior to all other English poets, including Shakespeare. Very early on, though, he was championed by Whigs, and decried by Tories: with the regicide Edmund Ludlow he was claimed as an early Whig, while the High Tory Anglican minister Luke Milbourne lumped Milton in with other "Agents of Darkness" such as John Knox, George Buchanan, Richard Baxter, Algernon Sidney and John Locke. The political ideas of Milton, Locke, Sidney, and James Harrington strongly influenced the Radical Whigs, whose ideology in turn was central to the American Revolution. Modern scholars of Milton's life, politics, and work are known as Miltonists: "his work is the subject of a very large amount of academic scholarship".