William Steuart qualified as an electrical engineer in England and was admitted as a member of the (British) Institution of Electrical Engineers (MIEE). Afterwards he decided to take up medicine and qualified in London in 1911 as a member of the Royal College of Surgeons of England (MRCS) and a licentiate of the Royal College of Physicians of London (LRCP). In 1913 he was appointed medical officer in charge of the X-ray department at the Johannesburg General Hospital. Later he became radiologist to the Miners' Phthisis Medical Bureau of the South African Institute for Medical Research and lecturer in radiology at the University of the Witwatersrand.
Following an outbreak of poliomyelitis in South Africa in 1917, Steuart designed and constructed an experimental apparatus for inducing sustained artificial respiration in children suffering respiratory failure as a result of poliomyelitis. He described his 'iron lung' or tank respirator and demonstrated the principle upon which it was based at a meeting of the Witwatersrand Branch of the British Medical Association. An account of the meeting was published under the title 'Demonstration of apparatus for inducing artificial respiration for long periods' in the Medical Journal of South Africa in March 1918. As the polio epidemic had died down by this time, the machine was not immediately needed and its whereabouts are unknown. Steuart was a self-effacing man who shunned publicity and his experiments were largely forgotten. The invention of the iron lung is still usually attributed to Phillip Drinker, an American public health engineer, in 1929.
Steuart's other contributions to medical science included papers on 'A method of treating carcinoma of the oesophagus' (Archives of the Roentgen Ray, 1913) and 'Radiography in its relation to miners' phthisis on the Witwatersrand' (Archives of Radiology and Electrotherapy, 1923); his collaboration with W. Watkins-Pitchford* and A.J. Orenstein* on 'A preliminary enquiry into the prevalence of pulmonary tuberculosis amongst mine natives' (Medical Journal of South Africa, 1916) and with A.H. Watt, L.G. Irvine* and J. Prat Johnson on 'Silicosis on the Witwatersrand' (South African Medical Record, 1916).
In 1933 Steuart lived in River Street, Lower Houghton, and was the senior partner in a medical practice in Jeppe Street which included Dr H.N. Krige and Dr R.J.W. Charlton. He was married to Ida Kathleen Frances Aicken, but they were divorced in 1929. His second wife, Jessica Cartridge, died in 1930. Afterwards he married Muriel Freeman, who survived him. He had at least two sons and two daughters.