William T. Thiselton-Dyer was the son of William George Thiselton-Dyer, a medical practitioner. He first studied medicine for a while at King's College, London, and then entered Christ Church College, Oxford, to study in the natural sciences. He eventually obtained the degrees Bachelor of Science (BSc), Master of Arts (MA) and Doctor of Philosophy (PhD). After holding appointments as professor of natural history at the Royal Agricultural College, Cirencester (1868), and professor of botany at the Royal College of Science, Dublin (1870), he was appointed professor of botany to the Royal Horticultural Society, London, in 1872. There he met Sir Joseph Hooker*, director of the Royal Botanical Gardens at Kew, near London, who got him to collaborate on compiling the Flora of British India. In 1875 he became assistant director of Kew Gardens, with responsibility for the botanical affairs of the British colonies. In this position he sent out plants and seeds to the colonies, as well as gardeners who assisted in the cultivation of economically important plants. For example, the South American rubber trees that he sent to Ceylon led to the development of extensive rubber plantations in the East. In a paper entitled "The botanical enterprise of the Empire", read at the Colonial Institute in 1880, he provided an overview of botanic gardens throughout the British Empire, including those in Cape Town, Port Elizabeth, Grahamstown, Graaff-Reinet, King William's Town, Queenstown, and Durban. In 1877 he married Harriet Ann Hooker, eldest daughter of Sir Joseph Hooker, with whom he had one son and one daughter. She was an amateur artist of skill who contributed many plates to Curtis's Botanical Magazine and became a prominent figure in English botanical circles.
Thiselton-Dyer succeeded Hooker as director of Kew Gardens in 1885 and held this position for 20 years. During his term of office the gardens and herbarium were further expanded and the Jodrell Laboratory for botanical research was established. One of his major achievements was to continue publication of the Flora Capensis. Work on this ambitious project had stopped with the death of W.H. Harvey* after only three volumes had been published during 1859-1865. Work was resumed in 1877 with the collaboration of both South African and British botanists (including H. Bolus*, E.P. Phillips*, Miss E.L. Stephens*, N.E. Brown*, and J.G. Baker*) and financial support from the governments of the British colonies in South Africa. Thiselton-Dyer edited the contributions, which were published in Volumes 4-7 (Dublin, 1896-1925). A supplement to Volume 5, Section 2, edited by Sir Arthur W. Hill, was added in 1933. The work contains descriptions of over 11 000 species of South African flowering plants found south of the Tropic of Capricorn. It remained the only standard work on its subject for several decades. Considering that most of the collaborators were staff members at Kew who relied solely on dried specimens, the work was of a surprisingly high (though somewhat uneven) standard.
As an expert and hard-working systematist, Thiselton-Dyer was the leading figure in British botany of his time. In addition to the Flora Capensis he also edited some other works: Volumes 4-8 of the Flora of Tropical Africa (London, 1898-1913), written by Daniel Oliver and others; the journal Icones Plantarum (1896-1906); the Annals of Botany (1887-); and the Botanical Magazine (1905-1906). He furthermore initiated the Kew Bulletin of Miscellaneous Information in 1887. As co-author of Alfred R. Wallace he helped to write The distribution of life (New York, 1885) and as co-author of H. Trimen contributed to the Flora of Middlesex (London, 1869). His numerous papers dealt with plant anatomy, descriptions of plant collections, evolution, plant diseases, and other botanical topics. Among those that relate to southern Africa are "The tree aloes of South Africa" (Nature, 1874); "On the perigynium and seta of Carex" (Journal of the Linnean Society (Botany), 1875); "On the genus Hoodia" (Ibid, 1876); "The carpophyll of Encephalartos" (Annals of Botany, 1901); "Protective adaptation" (Ibid, 1906); and descriptions of species such as Stapelia olivacea, Hoodia gordoni, Hoodia bainii and Encephalartos villosus.
Thiselton-Dyer was a Royal commissioner for the Melbourne Centennial Exhibition (1888), the Paris International Exhibition (1900), and the St Louis Exhibition (1904), and botanical advisor to the Secretary of State for the colonies from 1902 to 1906. After retiring in 1905 he settled in Witcombe, Gloucestershire. One of his hobbies was the identification of plants mentioned in the works of classical authors such as Virgil and Theophrastus. He also made a study of the South African cycads, but did not complete a publication on this work. He was honoured as a Knight Commander of the Order of St Michael and St George (KCMG, 1899) and as a Companion of the Order of the Indian Empire (CIE), and awarded honorary Doctor of Laws (LLD) and Doctor of Science (DSc) degrees. He was elected a Fellow of the Royal Society of London in 1880, serving as vice-president in 1896/7, was also a Fellow of the Linnean Society, and in 1912 was elected an Honorary Fellow of the Royal Society of South Africa. In 1871 he became a member of the British Association for the Advancement of Science, serving on its council for many years and as president of Section D (Zoology) in 1888 and of Section K (Botany) in 1895.