Thomas was educated at St Mungo's College, and Anderson's College and University, Glasgow, graduating M.B., Ch.B. in 1906, and M.D. in 1910. He also took the Diploma of Public Health in 1908, and took up the posts of resident medical officer of Enfield and Edmonton Joint Isolation Hospital; of assistant medical officer of health and school medical officer to the Ayrshire County Council, and medical officer of health, bacteriologist, medical superintendent of the Infectious Diseases Hospital, and medical officer of the tuberculosis dispensary to the Barnes Urban District Council. On 12th April 1915, when he joined the R.A.M.C., he was medical officer of health and school medical officer of Heston and Isleworth, and was consulting physician to the local isolation hospital committee. He gained promotion to Captain on completion of one year’s service. Thomas was the youngest son of William and Margaret Strain of Buarbrae, Wishaw; and the husband of Minnie Strain of 2 Southwood Court, Golders Green, London.