A.D. Thomas emigrated to South Africa as a small boy with his parents and grew up on a farm in the northern Transvaal (now Limpopo). After World War I (1914-1918) he was employed as a temporary stockman at Armoedsvlakte, near Vryburg, shortly after its inauguration as a research centre. Here he soon became a laboratory assistant to Sir Arnold Theiler*. He served in this capacity also at Allerton (near Pietermaritzburg) and Onderstepoort before starting his veterinary studies, qualifying (BVSc) at the University of Pretoria in 1926. He was then appointed to the research staff at Onderstepoort, also lecturing in pathology in the Veterinary Faculty. In 1928 he contributed three papers to the South African Journal of Science, dealing with "Several cases of so-called bent-leg in sheep", "Gross cysticercus invasion of the liver in lambs (Hepatitus cysticercosa)", and " Emphysema of the rumen in a sheep".
Thomas was awarded the DVSc degree by the University of South Africa in 1929 with a thesis on Skin cancer in the Angora goat in South Africa and was promoted to professor of pathology, a position he retained until his resignation in 1946 to start a practice in Pretoria. He published some 30 articles on a variety of pathological subjects, ranging from nutritional deficiencies to infectious diseases such as rabies and bluetongue. In 1955 he sold his practice and rejoined the Division of Veterinary Services in order to establish a Veterinary Investigation Centre in Louis Trichardt. Here he was the first to discover (with W.O.Neitz*) brown tick toxicosis in cattle. After retiring in 1965 he settled on a farm in the same district, later (1980) moving to George where he remained until his death.