John Thomas Dunston (sometimes wrongly Dunstan) qualified at Guy's Hospital, London, in 1899 as a member of the Royal College of Surgeons (MRCS) of England and a licentiate of the Royal College of Physicians (LRCP) of London. The next year he was awarded the degree Bachelor of Surgery (BS) by the University of London and in 1901 the degree Doctor of Medicine (MD) by the same institution.
In August 1905 Dunston was appointed first assistant medical officer at the Pretoria Lunatic Asylum (later the Mental Hospital, Pretoria, and then Weskoppies Hospital). He later became medical superintendent of the hospital and remained there for most of his career. However, in 1914 he was appointed as medical superintendent of Valkenburg Mental Hospital, Cape Town, and acting inspector of asylums in the Cape Province. Two years later he was appointed as commissioner of mentally disordered and defective persons for the Union of South Africa, a position he held until his retirement in 1931. Over the years he served also as medical inspector of leper asylums, neurologist to Pretoria General Hospital, and from 1924 lecturer in clinical psychiatry and later professor of psychiatry and mental hygiene at the Medical School of the University of the Witwatersrand until 1934. From 1926 or earlier he also practised privately in Pretoria, and from 1933 also as a "nerve specialist" in Johannesburg.
Two of Dunston's early papers, published in the Transvaal Medical Journal, dealt with "Some considerations regarding the advisability of asylum treatment" (1906/7) and "Progressive systematised delusional insanity and its importance, with illustrative cases" (1908/9). His later papers included "The Mental Disorders Act, 1916 (Union of South Africa) in some of its medical aspects" (South African Medical Record, 1916); "The problem of the feeble-minded in South Africa" (Journal of Mental Science, 1921); "Retarded and defective children; native mentality; mental testing" (Report of the South African Association for the Advancement of Science, 1923), being his presidential address to Section F of the association; "Clinical responsibility in relation to mental disease" (with R.F.A. Hoernle, South African Medical Record, 1926); and "Sterilisation of the unfit" (South African Medical Journal, 1932). A lecture by him delivered under the auspices of the Pretoria Athenaeum Society was published in the Pretoria News of 30 April 1918 and later reprinted as a pamphlet titled Mental defectives and social problems (Cape Town, 1922, 7p), with a separate version in Afrikaans. A paper he read before the Triennial Congress of the South African Prisoners' Aid Association at Kimberley on 23 August 1923 was also published as a pamphlet, The relation of psychiatry to delinquency (Pretoria, 1923, 10p).
By 1917 Dunston was a member of the Royal Society of South Africa. He was married to Marie Reinette Dunston, born Jeppe.