MacArthur, Sir William Porter (1884–1964), army medical officer, Irish-language scholar, and historian, was born 11 March 1884 at Belmont, east Belfast, Co. Down, the only son of John Porter MacArthur (d. 1930), tea merchant, and his wife Margaret Rainey (d. 1936), daughter of William Baird of Donemara, Co. Donegal. His father's family were descended from Scottish settlers who had emigrated from Argyll in the eighteenth century. When he was a child, the family moved to Bangor, Co. Down, where he attended Dr Connolly's school before entering QCB to study medicine. He had been interested in the Irish language since his childhood years and, as his family had holidayed in the Donegal Gaeltacht, he became fluent in the language and attended the Belfast feis of 1902. While studying at Queen's he actively promoted the study of Irish, and in 1906 was one of the founding members of QCB's Gaelic Society. He remained active in Irish-language scholarship for the rest of his life. Graduating MB and B.Ch. (1908), he was commissioned as lieutenant in the RAMC in 1909. Further studies followed and he subsequently graduated DPH from the University of Oxford (1910) and MD from QUB (1911).
Posted to Mauritius (1911), he engaged in an intensive study of tropical diseases, and this field of medicine became his speciality. Promoted to captain (July 1912), he was elected FRCPI (1913) and remained in Mauritius till 1914, gaining much experience in treating typhoid fever, for which he developed his own vaccine. After the outbreak of the first world war, he was posted to the British Expeditionary Force in France (1915) and was wounded in the stomach during the Somme offensive (1916). Awarded the DSO and mentioned in dispatches, he was invalided home and saw no further wartime service. In 1919 he was awarded an OBE and appointed to command the army's school of hygiene in Blackpool. He then took a course in entomology at the London School of Tropical Medicine, receiving a diploma in tropical medicine and hygiene from Cambridge (1920). Appointed professor of tropical medicine at the RAMC college in London (1922), he held this appointment till 1929 and introduced a course in entomology. He served as consulting physician to the army (1929–34) and again as professor of tropical medicine at the RAMC college (1932–4) before being appointed to the war office, where he served as deputy director-general of the army medical services. Appointed honorary physician to George V (1930), he served in this capacity to both Edward VIII and George VI, remaining as a royal physician till 1941. In 1935 he was appointed director of studies and commandant of the RAMC college (1935–8). Though he devoted much of his time to teaching, he continued to carry out his own research, conclusively proving the link between cysticercus infection by the pork tapeworm, Taenia solium, and some cases of idiopathic epilepsy.
In 1938 he was promoted to lieutenant-general and appointed director general of the army medical services. He was made a CB (1938) and KCB (1939), yet his period as director general was not a happy one for him. He disliked the bureaucracy of the War Office and the administrative duties of the post, much preferring to teach and carry out medical research. He realised that war with Germany was inevitable, and his notable achievement was to stockpile a supply of medical equipment which would then be on hand at the outbreak of war. While he was criticised for this by a parliamentary subcommittee, it was ultimately shown that he had acted wisely. After the outbreak of the war, he defended the RAMC against the criticism of the press and the commons. In 1941, after he had suffered an arm injury and the pressure of work began to affect his health, he retired.
He embarked on a second career as a college lecturer, being appointed as a faculty member and lecturer in tropical medicine at the University of Oxford. A consultant surgeon at the Royal Masonic Hospital, London, he served (1943) on a committee of the NI regional hospitals council, carrying out a survey of the region's hospitals. He also served as an examiner for the universities of Oxford, Cambridge, Liverpool, and London, and at the Royal College of Surgeons in London. In 1946 he was made a colonel commandant of the RAMC; he was also the editor of Transactions of the Royal Society of Tropical Medicine and Hygiene.
Throughout his medical career, he had continued to immerse himself in Irish-language studies. Summer holidays were spent in the Donegal Gaeltacht and he also travelled to Scotland, noting the similarities in the two Celtic languages. He remained active in the Queen's Gaelic Society and was the guest of honour at the society's golden jubilee celebrations (1956). Interested in historical epidemics, he had a knowledge of both Latin and Old English and carried out research into various medical calamities of the past. He wrote articles on historical medical subjects including the plague of Athens (430 BC), leprosy in medieval England, and the black death. Such articles included ‘Some medical references in Pepys’ in Ir. Jn. Med. Sc. (1928), ‘The identification of some pestilences recorded in the Irish annals’ in IHS (March 1949), and ‘The pestilence called “scamach” as mentioned in the Annals of Ulster’ in IHS (March 1951). He carried out extensive research into the diseases that prevailed during the famine and published the results as ‘Medical history of the famine’ in R. D. Edwards and T. D. Williams (ed.) The great famine in Irish history, 1845–52 (1956). In 1960 he published The Appin murder and also contributed to the British encyclopaedia of medical practice and the supplemental volumes of the DNB. He received numerous honours for historical work, including the Arnott medal (1929), the Chadwick medal (1935), the Robert Campbell medal (1951) and the Scott-Heron medal (1957). Awarded honorary degrees by QUB (1935) and Oxford (1949), he was elected as the Vicary lecturer at the Royal College of Surgeons, London (1931), and the Honeyman Gillespie lecturer at Edinburgh University (1955). He died in Chiswick, London, on 30 July 1964. His remains were returned to Ireland and were buried in the cemetery in Balmoral.
He married (1914) Marie Eugénie Thérèse, third daughter of Louis Ferdinande Antelme, MD, of Mauritius. They had two sons, Colàn MacArthur, writer and director of the Rand Organisation, and Ian MacArthur, elected as MP for Perth and East Perthshire in 1959. There is a large collection of his papers in the RAMC archives, Keogh Barracks, Aldershot.