John D. Borthwick qualified as a member of the Royal College of Veterinary Surgeons (MRCVS) at the New Veterinary College, Edinburgh, in 1888. He arrived in the Cape Colony early in 1889 and in March that year was appointed as the first assistant veterinary surgeon to serve under chief veterinary officer D. Hutcheon*. During 1891 to 1893 he was seconded to assist A. Edington* at the Colonial Bacteriological Institute in Grahamstown, and afterwards served at different places in the Colony. In 1897 he worked on rinderpest duty in various districts. In 1898 he was posted to Somerset East, where he remained until he succeeded Hutcheon as chief veterinary officer of the Cape Colony in July 1906. After the Union of South Africa was formed in 1910 he became assistant principal veterinary officer (effective January 1912), and was stationed in Pretoria. In May 1921 he succeeded C.E. Gray* as principal veterinary officer of South Africa, until his retirement in March 1927.
In 1895 Borthwick showed that regular feeding of bone meal greatly reduced the incidence of lamsiekte (botulism) in cattle, and published an article on the disease in the Agricultural Journal of the Cape Colony the next year (Vol. 9, pp. 141-144). In his report for 1898 from Somerset East he described some experiments on "krimpsiekte" (Cotyledon poisoning). In 1922 he produced a short pamphlet on anthrax which was published by the Department of Agriculture, while he also reported anually on "Glanders" and "Epizootic lymphangitis" in the Agricultural Journal of the Union of South Africa during the early nineteen-twenties. However, his main interest was animal husbandry, including the breeding of mules. Hence in 1903 he went to Spain to buy Catalonian Jacks for breeding improved mules, and in 1911 bought Hereford cattle and horses in England. He also published an article on ‘Tick paralysis affecting sheep and lambs’ in the Veterinary Journal.(London) in 1905.
Borthwick became a member of the Veterinary Medical Association of the Cape of Good Hope in 1906, and served as its president from 1908 to 1913. Upon his retirement after 38 years of service a commentator referred to his "yeoman service in the control of animal disease in South Africa, and in building up an effective field organisation" (Notes and news, 1927). After his retirement he was employed as veterinary adviser by the Imperial Cold Storage Company, interesting himself particularly in the transport of slaughter stock by rail.
Borthwick was married to Elizabeth Edith Walton, who was a psychiatric patient at the time of his death.