British botanist educated at Enfield, who later studied medicine at Middlesex Hospital. He settled in Cavendish Square with his elder brother, the naturalist Edward Turner Bennett. J.J. Bennett was employed as an assistant (1827) by the Department of Botany at the British Museum under Robert Brown. Together they published Plantae Javanicae Rariores (1838-1852) based on the collections of Thomas Horsfield (1773-1859), types of which are at BM. Bennett subsequently became Brown's successor at BM as the second Keeper of Botany (1859-1871).

Brown bequeathed his personal herbarium to Bennett in 1858, but this was later acquired by the museum (1876) on Bennett's death. A British herbarium compiled by Bennett, comprising some 500 specimens, was given to R.R. Hutchinson, later donated to the Croydon Natural History Society (CYN) and on long-term loan to SLBI since 1983. Bennett was elected fellow of the Linnean Society (1828) and of the Royal Society (1841). Several genera are dedicated to him including Benettia Gray (1821), which also commemorates his brother E.T. Bennett, and Benettia Miq. (1857) in the Flacourtiaceae (= Bennettiodendron Merr.).