William Morton Hewetson qualified as Bachelor of Medicine (MB) and Bachelor in Surgery (Ch B) at the University of Edinburgh in 1905 and obtained the Diploma in Public Health at the University of Birmingham. By 1908 he was attached to the North Midland Mounted Brigade Field Ambulance and in December that year achieved the rank of Lieutenant. However, by 1910 he had moved to Southern Rhodesia (now Zimbabwe) and settled in Wankie (now Hwange) as medical officer to the Wankie Colliery Company. By 1918 he had moved to Sinoia (now Sinhoyi), where he was government medical officer of the Lomagundi District.
Hewetson became a member of the South African Association for the Advancement of Science in 1911. In January that year he delivered an address before the Rhodesia Scientific Association in Bulawayo on "The causation and prevention of scurvy, with special reference to pneumonia", which was published in the society's Proceedings (1911, Vol. 11, pp. 5-36). He also published a paper on "The etiology of scurvy" in the Transvaal Medical Journal (1910/11, Vol. 6, pp. 183-193 and 207-215), but in a critical review of the paper (pp. 242-243), presumably by the editor, it was alleged that he appears to have confused scurvy with an inflammation of the gums, know as scorbutic gums, which was a local seasonal epidemic disease.
In May 1922 Hewetson addressed the branch of the Rhodesia Scientific Association in Salisbury (now Harare) on "Environmental influences affecting blondes in Rhodesia and their bearing on the future. A survey of the situation from the medical and scientific standpoint". The paper dealt with the effects of the natural environment on light-skinned persons, and was published as a pamphlet (27p). Four years later he delivered another address in Salisbury, on "Race contact in Southern Rhodesia; some fundamental considerations" (Salisbury, 1926, 24p). He also published a paper on the etiology of blackwater fever in the Journal of Tropical Medicine and Hygiene (1924/5).
Hewetson appears to have returned to England around 1927. From that time onwards several of his letters to the editor were published in the British Medical Journal, dealing with "Quinine in obstetric practice" (1928), "Quinine in albuminuria" (1930), "Use of quinine in normal labour" (1933), "The quality of sunlight" (1935), "A bureau of geographical medicine" (1936), and "Atmospheric ionization and influenza" (1936).