He was born at Iford Manor, Wiltshire, and educated at Hyde Abbey School, Winchester before entering the University of Oxford in 1797, becoming successively student and tutor of Christ Church. In 1811, he was appointed Regius Professor of Greek in the University. Taking orders, he held (1815–1847) the college living of Westwell, Oxfordshire, and other ecclesiastical preferments simultaneously with his professorship. From 1831 until his death, he was Dean of Christ Church.
As curator of the Bodleian Library and principal delegate of the Oxford University Press, Gaisford was instrumental in securing the cooperation of distinguished European scholars as collators, notably Bekker and Dindorf. Among his numerous contributions to Greek literature may be mentioned, Hephaestion's Encheiridion (1810); Poëtae Graeci minores (1814–1820); Stobaeus' Florilegium (1822); Herodotus, with variorum notes (1824); Suidas' Lexicon (1834); Etymologicum Magnum (I848). Eusebius's Praeparatio evangelica (1843) and Demonstratio evangelica (1852). In 1856, the Gaisford prizes, for Greek composition, were founded at Oxford to perpetuate his memory.
On 23 June 1843, his 21 year-old son, William Gaisford, drowned while swimming in the river Thames at Sandford Lock - a notoriously dangerous spot. He got into difficulties and his friend, Richard Philmore, entered the water to save him, but both young men perished. They are buried in Christ Church Cathedral. They are commemorated by an obelisk at Sandford Lock and two memorial tablets in the north walk of the Cathedral cloisters.
The Gaisford Prize was founded in Gaisford's honour, shortly after his death.
- "Nor can I do better, in conclusion, than impress upon you the study of Greek literature, which not only elevates above the vulgar herd, but leads not infrequently to positions of considerable emolument." –Th. Gaisford, Christmas Day Sermon in the Cathedral, Oxford (Rev. W. Tuckwell, Reminiscences of Oxford, 2nd ed., 1907, p. 124)
- Michael Popkin (2001). "Brave Deeds and Tragedies". OXFORD INSCRIPTIONS - Inscribed Stones and Plaques in Oxford. Retrieved 2009-11-27.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.