David Thomas Mitchell qualified as a veterinarian (MRCVS) at Dublin in 1908 and that same year emigrated to the Transvaal Colony. He was first employed as a district veterinarian. In June 1910 he was appointed as assistant veterinary bacteriologist at the Onderstepoort Veterinary Research Institute and two years later his post was changed to veterinary research officer. During this period he worked on various stock diseases and reported on the symptomatology and pathology of lamsiekte in the Second Report of the Director of Veterinary Research (1912, pp. 161-180). He also published a paper on "The effects of snake venom on domestic animals, and the preparation of anti-venom serum" in the Report of the South African Association for the Advancement of Science (1915, pp. 337-354). In 1914 he was transferred to Natal in charge of the Allerton Laboratory, where he was promoted to senior veterinary research officer in 1917. The next year he returned to Onderstepoort, was promoted to assistant director of veterinary research in 1919, and acting director during the absence on leave of R.E. Montgomery*. That year he wrote two papers dealing with cattle poisoning: "Poisoning of cattle by feeding on Paspalum dilatatum infected with Claviceps paspali" and "Poisoning of cattle by Diplodia-infected maize". Both papers were published in the Report of the South African Association for the Advancement of Science (1919, pp. 391-395 and 446-452).
Mitchell became a member of the Transvaal Veterinary Medical Association in 1909 and served as its honorary secretary in 1919/1920. When the association became the South African Veterinary Medical Association in 1920 he continued serving as honorary secretary, at least for the first year. He became a foundation member of the South African Biological Society in 1916/7.
When Sir Arnold Theiler* was re-appointed in 1920 Mitchell was transferred to the Armoedsvlakte research station near Vryburg and after a year's study leave overseas returned to Allerton in 1923 for a second tour of duty. In 1926 he reported on the toxic effects on ruminents of Urginea macrocentra (locally known as the Bergtulp) in the 11th and 12th Report of the Director of Veterinary Education and Research; eight years later he and A.S. Canham* followed this up with an article on the toxic effect of a related species, U capitata, in the Onderstepoort Journal of Veterinary Science (1934, Vol. 2, pp. 681-689). He also published several articles on the diseases of domesticated birds in the Journal of the Department of Agriculture (1925) and Farming in South Africa (1925, 1928).
In 1929 Mitchell left Allerton on secondment to the government of Burma (now Myanmar) to study anthrax in elephants. He resigned his South African post to become the Burmese director of veterinary services. He was awarded a silver plate in that country for his successful use of the Onderstepoort anthrax vaccine to control the disease in elephants. In 1939 he returned to South Africa and was sent to Tanganyika (now Tanzania) to supervise rinderpest vaccine production. On his return to Onderstepoort in 1940 he studied virus diseases until he joined the South African Institute for Medical Research in February 1943 to produce typhus vaccine. He died that same year. At some time he was honoured as a Member of the Order of the British Empire (MBE). He was survived by his wife, Kathleen E. Mitchell, born Ritchie, with whom he had two daughters and a son.