Thomas was educated at Glasgow University, where he qualified M.B. and C.M. with commendation in 1884. He joined the Army Medical Services on 5th February 1887, becoming Major on 5th February 1899. He served in the third China war in 1900, when he was mentioned in dispatched in the London Gazette on 13th September 1901, and received the medal. From 1st May 1902 to 25th April 1906, he was Deputy Assistant Director-General of the A.M.S., and in 1906 he served as a member of the committee for the investigation of Mediterranean fever. He was promoted to Lieutenant Colonel on 29th July 1911. Thomas was a Knight of Grace of the Order of St. John of Jerusalem in England. He was re-employed during the Great War but died suddenly of heart disease at the Alexandra Hospital, Cosham, Hants. He was the husband of C M McCulloch of 24 Ramsbury Road, St. Albans.