G. Ritchie Thomson, son of John Alexander Thomson and his wife Isabella, born Ritchie, qualified as Bachelor of Medicine (MB) and Master in Surgery (CM) at the University of Edinburgh in 1887 and also attended the universities of Glasgow and Halle, Germany. After an internship at the Glasgow Royal Infirmary he became resident medical officer at Tottenham Hospital in north London. In 1891 he returned to Glasgow and was appointed assistant surgeon at the Royal Infirmary and the Royal Hospital for Sick Children. After serving as surgical specialist during the Anglo-Boer War (1899-1902) he settled in Johannesburg in private practice, was registered to practice medicine in the Transvaal in 1902, and was elected president of the Transvaal Medical Society that same year. A few years later he was a surgeon at the Johannesburg General Hospital and published several articles in the Transvaal Medical Journal: "The diagnosis and treatment of fractures of the long bones" (1907/8, Vol. 3); "Notes on a case of traumatic diaphragmetic hernia" and "Remarks on Bier's hyperaemia as a remedy" (1908/9, Vol. 4). He was a member of the South African Association for the Advancement of Science in 1906, but his membership had lapsed by 1910.
In 1914 Thomson was a member of the Transvaal Medical Council. He was again on active service during World War I (1914-1918), first in the South West Africa campaign and later in command of the First South African Hospital in Abbeville, France, and attained the rank of lieutenant-colonel in the South African Medical Corps. After the war he returned to private practise in Johannesburg, with an honorary appointmnt at the Johannesburg General Hospital. In 1919 he was honoured by Britain as a Companion of the Order of St Michael and St George (CMG). The next year he was appointed as the first professor of surgery at the newly established Medical School of the University of the Witwatersrand, a position he held until his retirement in 1930. During this period he was also the senior honorary surgeon at the Johannesburg General Hospital. He was elected a Fellow of the Royal Society of Edinburgh in 1926. The University of the Witwatersrand awarded him an honorary Doctor of Laws (LLD) degree in 1931.
Thomson remained a British citizen and after his retirement returned to Scotland, where he practised as a surgeon at Edinburgh and Tobermory, Isle of Mull. He was married to Mabel Alice Powys, who died on 5 November 1942. They had a son and two daughters.