James Walker qualified (MRCVS) in Edinburgh, Scotland in 1896 and that same year emigrated to South Africa. He joined the Cape Mounted Rifles as a military veterinary surgeon in 1897 and served in the Anglo-Boer War (1899-1902). After the war, in January 1903, he joined the Veterinary Department of the Transvaal Colony and was stationed at Ermelo as district veterinary surgeon. There, in 1907, he was the first to notice the existence of gousiekte. During 1908 and 1909 he carried out feeding experiments that implicated a plant poison, but the causal species (Pachystigma pygmaeum) was not specifically tested. The results were reported in the Report of the Government Veterinary Bacteriologist (Transvaal) for 1908/9 (pp. 74-99). Meanwhile Walker had been appointed assistant government veterinary bacteriologist at the Daspoort (Pretoria) Research Station in July 1908, to assist Walter Frei*. Later that year he transferred to the newly established Onderstepoort Veterinary Research Laboratory, where he worked until 1918 (from 1910 as veterinary research officer). His research during this period covered a wide range of subjects, including lamsiekte, bovine piroplasmosis (redwater), gallsickness, dourine (slapsiekte) and other animal diseases. A comprehensive report on the first of these diseases, "Investigations into the disease lamziekte in cattle", was included in the Second Report of the Director of Veterinary Research (Union of South Africa) in 1912 (pp. 79-160). Among other contributions he definitely showed in 1915 that aspergillosis in the ostrich chick is caused by Aspergillus fumigatus.
Walker served on the council of the Transvaal Veterinary Medical Association from 1914 to 1918. He became a foundation member of the Transvaal Biological Society in 1907, joined the South African Association for the Advancement of Science in 1912, and was a member also of the Royal Society of South Africa.
In June 1918 Walker assumed duty as veterinary pathologist at the Kabete Veterinary Research Station near Nairobi, Kenya, and in 1922 became chief veterinary research officer in that country. Five years later he was honoured as an Officer of the Order of the British Empire (OBE) for his services. His ideal was to develop the Kabete laboratory into a Central African research institute equal to Onderstepoort, but he was unable to achieve this. While in Kenya he published two papers in the South African Journal of Science, "The application of the conglutination reaction to the serum diagnosis of bovine pleuropneumonia (Lung sickness)" (1923) and "Observations on the nature of the immunity conferred to East Coast Fever by natural infection, or exposure thereto, in inoculated and non-inoculated cattle" (1926). In 1932 he acted as deputy director of animal industry in Kenya, and was awarded the Dr Med Vet degree by the University of Zurich for his thesis East African Swine Fever. The next year he returned to South Africa and undertook research on foot and mouth disease, including a study in present Botswana of the disease in calves. After his retirement he practiced in Johannesburg until his death in 1952. He was a genial and unpretentious person.