Jellett, John Hewitt (1817–88), mathematician and provost of TCD, was born 25 December 1817 at Cashel, Co. Tipperary, eldest among four sons and one daughter of the Rev. Morgan Jellett (d. 1831), rector of Pallasgrean, near Cashel, and Harriette Anne Jellett (née Poole; d. 1829), one of twelve sisters from Mayfield, Co. Cork. His father published (1820) a volume of anthems for church services. John was educated at Midleton College, and entered TCD 21 October 1833, having been placed first in the entrance examination; he was elected a scholar in 1836, and (after graduating BA, 1838) a fellow in 1840. In 1846 he was ordained a clergyman of the Church of Ireland. In 1847 he became first professor of natural philosophy in TCD, and published a number of scientific papers in the Proceedings of the RIA, of which he was president 1869–74; he received the academy's Cunningham medal in 1850. He was a commissioner of national education from 1868. He was a fellow of the Royal Society, and in 1881 was awarded the Royal Society's gold medal. He worked on polarised light, and to study what he called ‘chemical optics’ he developed a saccharometer, an instrument that exploited the properties of polarised light in the analysis of chemical compounds, especially sugars. He published A treatise of the calculus of variations (1850) and Treatise on the theory of friction (1872). Jellett also published sermons, religious essays, and his 1877 Donnellan lectures as The efficacy of prayer (1878). He gave up his professorship in 1870 and was coopted senior lecturer in 1871, a post he held until 1881. He was the only member of the board to support the admission of women as students to college (his own daughters were very carefully educated), and he supported educational and other charities in Dublin. With Samuel Haughton (qv), he edited the works of James MacCullagh (qv), which appeared in 1880.
Jellett was prominent for many years in church synods, particularly after disestablishment, when his advice and influence were often crucial in the formation of a new identity and structures; he tended towards the evangelical position, but sought always to preserve church unity. He received a DD from TCD in 1881 and a DCL (h.c.) from the University of Oxford in 1887. He had been a liberal in politics, and it was a liberal government in 1881 that endorsed his appointment as provost of TCD in succession to Humphrey Lloyd (qv). However, after Gladstone's attempt to introduce home rule in 1886, Jellett became a strong supporter of the union.
He married (7 July 1855) his cousin Dorothea Morgan of Tivoli, Co. Cork; they had three sons – William M. Jellett (1858–1936), lawyer and MP, and two sons who died in infancy – and four daughters, of whom one was Eva Josephine Jellett (qv). Another daughter married George Francis Fitzgerald (qv). Jellett's grandchildren included Mainie Jellett (qv) and Dorothea (‘Bey’) Jellett, who conducted the orchestra at the Gaiety theatre, Dublin. In February 1888 Jellett developed blood poisoning after a throat infection, and died in the Provost's House on 19 February 1888. His impressive funeral on 23 February was from the college chapel to Mount Jerome cemetery; the largest procession that had ever left TCD, it passed shuttered shops and houses, where blinds were drawn as a mark of respect. A portrait by Sarah Purser (qv) is in TCD.