The eldest son, but with five older sisters, Henry Longueville Mansel was brought up in Cosgrove Rectory, where his father held the living. He was a thoughtful but somewhat strange child, asking bizarre questions at an early age.

On 29 Sept. 1830 he entered Merchant Taylors' School, and was placed in the house of the head-master, J. W. Bellamy. He was irascible, though easily pacified, and cared little for games, but soon showed remarkable powers of concentration and acquisition. He had a very powerful memory, and spent all his pocket-money on books, forming ‘quite a large library of the English poets.’ He was already a strong Tory, as became a member of an old family of soldiers and clergymen. He wrote in the ‘School Magazine’ in 1832–3, and in 1838 published a volume of youthful verses, ‘The Demons of the Wind and other Poems.

In 1838 Mansel won the prize for English verse and a Hebrew medal given by Sir Moses Montefiore. In 1839 he won two of the four chief classical prizes, and on 11 June 1839 was matriculated as a scholar of St. John's College, Oxford. He was a model undergraduate, never missing the morning service at chapel, rising at six, and, until his health manifestly suffered, at four, and working hard at classics and mathematics, while at the same time he was sociable and popular. His private tutor for his last years was Archdeacon Hessey, who was much impressed by his thoroughness in attacking difficulties and his skill in humorous application of parallels to Aristotle, drawn from Shakespeare or ‘Pickwick.’ In the Easter term of 1843 he took a ‘double first.’ His vivâ voce examination is said to have been disappointing, because he insisted upon arguing against a false assumption involved in his examiner's first question.

On 16 Aug. 1855, Mansel married Charlotte Augusta, third daughter of Daniel Taylor of Clapham Common (she died in 1908 aged 83). He gave up teaching pupils and lived in the High Street at Oxford.

He continued to tutor and lecture to Philosophy students at St John’s.

From 1864 to 1868 Mansel was examining chaplain to the Bishop of Peterborough (Dr. Jeune). At the end of 1866 he was appointed by Lord Derby to the professorship of ecclesiastical history, vacant by the death of Dr. Shirley on 30 Nov. He delivered in the Lent term of 1868 a course of lectures upon ‘The Gnostic Heresies,’ published after his death.

In 1868 Mansel was appointed to the deanery of St. Paul's by Mr. Disraeli. His health was weakened by the pressure of business at Oxford, and he had been much distressed by the direction in which the university had been developing. He hoped to find more leisure for literary projects in his new position. There was, however, much to be done in arranging a final settlement with the ecclesiastical commissioners, and he was much occupied in finishing his share of the ‘Speaker's Commentary’ (the first two gospels) which he had undertaken in 1863.

He  also took the lead in promoting the new scheme for the decoration of the cathedral. ' I should wish to see such decorations introduced into St. Paul's as may give some splendour, while they would not disturb the solemnity or the exquisitely harmonious simplicity of the edifice ; some colour to enliven and gladden the eye, from foreign or native marbles, the most permanent and safe modes of embellishing a building exposed to the atmosphere of London. I would see the dome, instead of brooding like a dead weight over the area below, expanding and elevating the soul towards heaven. I would see the sullen white of the roof, the arches, the cornices, the capitals, and the walls broken and relieved by gilding, as we find it by experience the most lasting as well as the most appropriate decoration. I would see the adornment carried out in a rich but harmonious (and as far as possible from gaudy) style in unison with our simpler form of worship.” Mansel was a personal friend of Lord Carnarvon, who described him thus: “I remember, during part of a summer that I spent with him by the seaside, his characteristic determination to understand the method of sailing a boat, and the acuteness with which he resolved the practical details, as he got them from an old fisherman, into the more scientific principles by which they were really governed. I remember, on another occasion, the keen interest with which he learnt from a gamekeeper some of the mysteries of his craft in the rearing of birds; and though Dean Mansel would never have become a good pilot or gamekeeper, yet this keen interest in the occupations of others kept his own mind singularly fresh and active.” Mansel paid visits with his wife to his brother-in-law at Cosgrove Hall during his tenure of the deanery, and while staying there in 1871 he died suddenly in his sleep (30 July), from the rupture of a blood-vessel in the brain. A memorial window, representing the incredulity of St. Thomas, was erected to his memory in the north chapel of St. Paul's Cathedral, and unveiled on St. Paul's day 1879. St Paul’s cathedral staff told us that this window was destroyed by a bomb blast during WW2.

He paid visits with his wife (daughter of Admiral Moorsom) to his brother-in-law at Cosgrove Hall during his tenure of the deanery, and while staying there in 1871 he died suddenly in his sleep (30 July), from the rupture of a blood-vessel in the brain.

A memorial window, representing the incredulity of St. Thomas, was erected to Dean Mansel’s memory in the north chapel of St. Paul's Cathedral, and unveiled on St. Paul's day 1879. It was destroyed by bomb damage during the Second World War.

In the cathedral collections is a bust of Dean Mansel sculpted by E W Wyon. Catalogue number 5158. 77cm high.

Dean Mansel is also represented in the National Portrait Gallery.

Mansel’s philosophical teachings are difficult to understand. He maintained the purely formal character of logic, the duality of consciousness as testifying to both self and the external world, and the limitation of knowledge to the finite and "conditioned."

He applied metaphysical agnosticism to Christian theology. Consciousness, he held — agreeing with the doctrine of "natural realism" — implies knowledge both of self and of the external world.