Samuel Irvine Johnston, veterinary surgeon, qualified as a member of the Royal College of Veterinary Surgeons (MRCVS) at the Royal (Dick) Veterinary College, Edinburgh, in 1895. He came to South Africa in 1901, during the Anglo-Boer War (1899-1902), and for some time served as a veterinary officer in the 7th Hussars. When his regiment returned to England he obtained a position in the Veterinary Division of the Department of Agriculture of the Transvaal Colony, where he started work on 3 October 1905. By 1907 he was the government veterinary surgeon in Wakkerstroom. With regard to his military career, from November 1906 he served as veterinary lieutenant in the Eastern Mounted Rifles, and in November 1909 was promoted to veterinary captain in the Southern Mounted Rifles. He eventually reached the rank of major.
After the formation of the Union of South Africa in 1910 Johnston was appointed as a district veterinary surgeon in the Veterinary Division of the Union Department of Agriculture in April 1912. He was also appointed as captain in the South African Veterinary Corps Citizen Force in January 1915 and during World War I (1914-1918) served in East Africa and for some time acted as officer commanding of the Veterinary Hospital at Military Headquarters in Pretoria. In March 1918 he left the civil service and started in private practice in Johannesburg. He built up the Veterinary Vaccine Agency and on account of his extensive knowledge of horses often acted as a judge of horses at agricultural shows. He was a member of the council of the Witwatersrand Agricultural Society and one of the founders of the Johannesburg Polo Club.
Johnston was an active member of the Transvaal Veterinary Medical Association from its formation in 1903, serving on its council for 1917/8, and in 1920 became a member of its successor, the South African Veterinary Medical Association. In 1914 he became a member also of the Natal Veterinary Medical Association, having been sent there for a month in 1911. His only known publication is a short paper on "Thrombosis in the horse" (Journal of Comparative Pathology and Therapeutics, 1905, Vol. 18, pp. 78-79).