Alexander William Reid qualified as Bachelor of Medicine (MB) and Master in Surgery (CM) at the University of Aberdeen in 1894. At some time he also acquired the Diploma in Public Health of the University of Cambridge. After qualifying he became a surgeon in the Gold Coast (now Ghana), West Africa. He then went to India, where he worked as a civil medical officer, superintendent of gaol and health officer in the districts of Hoogly, Rungpore, Burdwan, and Puri. In 1903 he qualified as Doctor of Medicine (MD) at the University of Aberdeen with a dissertation entitled Observations on epidemics of cholera in India. That same year he came to the Colony of the Cape of Good Hope, where he was licensed to practice medicine on 10 December 1903. In April 1905 he married Henrietta May MacKenzie. Also in 1905 he was appointed medical officer of health for the district of Kimberley, a post he held until at least 1915. By 1926 he was assistant medical officer of health of Cape Town, residing in Claremont. He still held this post in 1930.
Reid published a book on Sanitation and public health, with special reference to conditions in South Africa (Cape Town, 1923, 270p., 2nd ed., 1927). By 1910 he was a member of the South African Association for the Advancement of Science, but his membership had lapsed by 1918.