Keble was born on 25 April 1792 in Fairford, Gloucestershire, where his father, also named John Keble, was vicar of Coln St. Aldwyns. He and his brother Thomas were educated at home by their father until each went to Oxford. In 1806, Keble won a scholarship to Corpus Christi College, Oxford. He excelled in his studies and in 1810 achieved double first-class honours in both Latin and mathematics. In 1811, he won the university prizes for both the English and Latin essays and became a fellow of Oriel College. He was for some years a tutor and examiner at the University of Oxford.
While still at Oxford, he was ordained in 1816, becoming a curate to his father and then curate of St Michael and St Martin's Church, Eastleach Martin, in Gloucestershire while still residing at Oxford. On the death of his mother in 1823, he left Oxford and returned to live with his father and two surviving sisters at Fairford.
Between 1824 and 1835, he was three times offered a position and each time declined on the grounds that he ought not separate himself from his father and only surviving sister. In 1828, he was nominated as provost of Oriel College but not elected.
Delivered on 14 July 1833, his famous Assize Sermon on "National Apostasy" gave the first impulse to the Oxford Movement, also known as the Tractarian movement. It marked the opening of a term of the civil and criminal courts and is officially addressed to the judges and officers of the court, exhorting them to deal justly. Keble contributed seven pieces for Tracts for the Times, a series of short papers dealing with faith and practice. Along with his colleagues, including John Henry Newman and Edward Pusey, he became a leading light in the movement but did not follow Newman into the Roman Catholic Church.
In 1835, his father died, and Keble and his sister retired from Fairford to Coln. In the same year he married Charlotte Clarke and the vicarage of Hursley in Hampshire, becoming vacant, was offered to him; he accepted. In 1836, he settled in Hursley and remained for the rest of his life as a parish priest at All Saints' Church.
Keble died in Bournemouth on 29 March 1866 at the Hermitage Hotel, after visiting the area to try and recover from a long-term illness as he believed the sea air had therapeutic qualities. He is buried in All Saints' churchyard, Hursley.
Keble has been described thus:
He was absolutely without ambition, with no care for the possession of power or influence, hating show and excitement, and distrustful of his own abilities.... Though shy and awkward with strangers, he was happy and at ease among his friends, and their love and sympathy drew out all his droll playfulness of wit and manner.... In personal appearance he was about middle height, with rather square and sloping shoulders, which made him look short until he pulled himself up, as he often did with 'sprightly dignity.' His head, says Mozley, 'was one of the most beautifully formed heads in the world,' the face rather plain-featured, with a large unshapely mouth, but the whole redeemed by a bright smile which played naturally over the lips; and under a broad and smooth forehead he had 'clear, brilliant, penetrating eyes which lighted up quickly with merriment kindled into fire in a moment of indignation.... a quiet country clergyman, with a very moderate income, who sedulously avoided public distinctions, and held tenaciously to an unpopular School all his life.
John Keble is remembered in the Church of England with a Lesser Festival on 14 July (the anniversary of his Assize Sermon) and a commemoration observed on 29 March (the anniversary of his death) elsewhere in the Anglican Communion.