T. Duncan Greenlees, son of a Scots medical practitioner, qualified as Bachelor of Medicine (MB) and Master of Surgery (CM) at the University of Edinburgh in 1882. He obtained experience of mental institutions in Britain as assistant medical officer to the City of London Asylum at Stone (1882-1884), the Counties Asylum at Carlisle (1884-1887) and the City of London Asylum at Dartford (1887-1890). He also became a prizeman of the (British) Medico-Psychological Association, was elected a fellow of the Royal Society of Edinburgh, and belonged to the British Medical Association and Caledonian Medical Society. His early publications included "Notes on a case of cerebral tumour" (American Journal of Insanity, 1885); "Cortical diseases and paralysis" (Ibid, 1886; "Contributions to diseases of the circulatory system in the insane" (Journal of Mental Science, 1886); "Studies of the sphygmograph among asylum patients" (Ibid, 1887); "Athetosis associated with insanity" (Brain, 1888); "Critical digest of general paralysis of the insane" (Brain, 1889); and "Notes on a case of traumatic epilepsy cured by operation" (Journal of Mental Science, 1892).
In August 1890 Greenlees came to the Cape Colony, where he was licensed to practice on 30 September that year. He succeeded Dr R. Hullah as surgeon superintendent of the Grahamstown Lunatic Asylum and visiting surgeon to the chronic sick hospital. In 1894 an Institute for the Care and Education of Weak-minded Children was attached to the Asylum, where mentally retarded children of European descent could be confined. From 1895 Greenlees was medical officer to this institution as well. He furthermore served as surgeon-captain to the Grahamstown First City Volunteers.
Greenlees was an excellent administrator and under his enlightened leadership the Asylum developed into a first-class institution. Soon after his arrrival he started publishing a quarterly magazine, the Fort England Mirror (the Asylum was housed in Fort England), in which he wrote a doctor's column reflecting a positive outlook on mental disease. His prominence in medical circles in South Africa was enhanced by several publications on local psychiatric matters. For example, his paper on "Criminal responsibility in the insane" appeared in the Cape Law Journal in 1892. On 8 May the same year he delivered a lecture in Grahamstown on "The brain", which was published in the Grahamstown Journal, and reprinted as a pamphlet. The next year he published "Poisoning by medicinal doses of bromide of potassium" in the (second) South African Medical Journal. "Statistics on insanity in South Africa" appeared in the American Journal of Insanity in 1894, followed by "Insanity among the natives of South Africa" in the Journal of Mental Science (1895). In the latter paper he suggested that the native brain and mental attributes are similar to those of European children; hence he expected his studies of insanity among natives and the microscopic anatomy of the native brain to provide new insights into the nature of insanity. Still in 1895 he wrote a speculative account of "The relation of genius to insanity" for the E.P. Magazine (Vol. 2(5), pp. 161-183), in which he argued strongly against the social stigma associated with insanity.
Greenlees qualified as Doctor of Medicine (MD) in 1901. Soon after the Anglo-Boer War (1899-1902) he contributed several further papers to the South African Medical Record: "Medical, social and legal aspects of insanity" (1903/4, Vol. 1, pp. 121- ); "The circulatory apparatus in general paralysis" (1904, Vol. 2, pp. 49- ); "Statistics of insanity in Grahamstown Asylum" (1905, Vol. 3, pp. 218- ); "Etiology, symptoms and treatment of imbecility" (1907, Vol. 5, pp. 17- ); and "Notes on some forms of mental disease" (1907, Vol. 5, pp. 257- ). Around this time he resided at Bathurst, where he rented a government cottage. Three photographs that he took of buildings in Bathurst and Port Alfred are in the Cory Library, Grahamstown.
Greenlees was a member of the Eastern Province Branch of the British Medical Association, serving as its president in 1897. In May 1896 he addressed members on the post-mortem appearances in a case of agromegaly, with an account of theories of the etiology of the disease. He was president again in 1903, delivering his presidential address on "Insanity - past, present and future", and thereafter served on its council. He also joined the Eastern Province Literary and Scientific Society and read a paper before it titled "On the threshold - studies in psychology" in June 1895. Two months later he was elected on the society's management committee. By 1898 he was a member of the South African Medical Association too. And in 1902 he became a foundation member of the South African Association for the Advancement of Science, though he did not remain with the association for long.
Greenlees retired at his own request in 1907, at the age of 49 years, and returned to England. There he assisted the council of the British Medical Association as the representative of its South African branches. He was appointed resident physician and superintendent of the Fenstanton Asylum at Streatham Hill, London, but around 1922 settled at St Leonard-on-Sea, in Sussex. There he became a well-known member of the medical profession in the Hastings district and served as a member of the central council of the British Medical Association for most of the period 1908 to 1925. He was a member of the Freemason organisation and a deacon in the Baptist Church. At his death he left a wife, a son and two daughters.