Dr James Thomas Bruce-Bays initially used the surname Bays. He studied in London and became a licentiate of the Society of Apothecaries (LSA) of London in 1887. The next year he obtained the degree Bachelor of Medicine (MB) and became a member of the Royal College of Surgeons (MRCS) of England. In 1889 he obtained the degree Doctor of Medicine (MD). He was sometime house physician and obstetrics officer at St. Mary's Hospital and house physician and house surgeon at the Hospital for Sick Children in London, and published two medical articles in The Lancet in 1889 and 1893.
Bays was licensed to practise in the Colony of the Cape of Good Hope on 16 May 1895. By 1897 he resided in Grahamstown and was acting district surgeon. In 1899 he was elected on the committee of management of the Albany Museum and during the same year he and a Mr. G. Randall donated two specimens of the lesser bushbaby (then named Galago moholi, now a subspecies of G. senegalensis) from Gaberones to the museum. At this time Bays was Medical Officer of Health for Albany, reporting on its water supply, sanitation, and other health matters to the Medical Officer of Health of the Cape Colony. He was also medical officer at Albany Hospital in Grahamstown from 1898 to 1911. Around the turn of the century he published three papers in the British Medical Journal on 'Dislocation of the ungual phalanx' (1897), 'Dislocation of the radio-carpal joint' (1899), and 'Bilharzia haematobia' (1901).
By 1908 he was using the surname Bruce-Bays and participated in the preparations for the annual meeting of the South African Association for the Advancement of Science, which took place in Grahamstown that year. He served on the local committee of the association, and was also honorary secretary of Section D (which then included bacteriology, physiology, and hygiene). The paper that he presented at the meeting was titled "The injurious effects of civilisation upon the physical condition of the native races of South Africa", in which he ascribed the high infant mortality in the locations to poor housing and food. In 1910 he published an article on "Food poisoning" in the South African Medical Record (Vol. 8, p. 131).
In 1911 Bruce-Bays was back in London, where he obtained the additional qualifications Bachelor of Surgery, and Doctor of Medicine in midwifery and the diseases of women. He had returned to South Africa by 1915 and settled in East London, where he still resided in 1926. He remained on the British Medical Register until 1953.