William Steward Mitchell D'Urban was a son of Lieutenant-General William James D'Urban, who took part in the British occupation of Natal as a major in 1842. He was thus a grandson of Sir Benjamin D'Urban, governor of the Cape Colony from 1834 to 1837. William Stewart Mitchell was a keen naturalist, though an attack of scarlet fever in 1849 had left him deaf. He spent much of his youth with his grandfather, first at Wynberg, Cape, and then in Canada. From 1856 to 1861 he published nine papers on the natural history of Canada in which he described his observations of birds and insects near Montreal, the natural history of Ottawa, a list of Coleoptera from near Montreal, Coleoptera collected near Hudson Bay, notes on Canadian Hymenoptera, and a catalogue of plants collected in the counties Argenteuil and Ottawa. He also listed his "Contribution to Canadian natural history" in the Report of the Geological Survey of Canada for 1858.
During 1860-1861 D'Urban spent a year in and around King William's Town while his father commanded troops in the Ciskei. In the preface to Volume 2 of the Flora Capensis (1862) he was thanked by W.H. Harvey* and O.W. Sonder* "for a collection of plants from British Caffraria, containing several rare species". With his parents he also collected butterflies and ferns. His observations led to two papers: "On the ferns of South Africa, and a comparison of them with the acrogenous plants of Great Britain and North America" (Reports and Transactions of the Devonshire Association for the Advancement of Science, Literature and Arts, 1863, Vol. 1, pp. 45-48) and "On the naturalized weeds of British Kafffraria" (Journal of the Linnaean Society (Botany), 1865, Vol. 8, pp. 267-272).
After leaving South Africa D'Urban settled in Devon, England, and later became the first curator of the Royal Albert Memorial Museum in Exeter. His plant specimens, including 593 sheets of ferns, were donated to the museum, but in 1952 the ferns were transferred to the British Museum (Natural History). However, his main collection of ferns is in Oxford. He was the first author of a book on The birds of Devon (with M.A. Mathew, 1892-1895), and papers on the molluscs (1879) and zoology (1880) of the Barents Sea. The butterfly genus Durbania was named after him by R. Trimen* of the South African Museum, Cape Town. In the preface to his South African butterflies... (1887-1889) Trimen acknowledged D'Urban's contribution of many butterflies from the East London - King William's Town region, with valuable accounts of their preferred habitat.