Thomas was educated at Kelso High School and Edinburgh University. First qualifying as a chemist he was for some years with Messrs Duncan Flockhart and Co of Edinburgh before entering on his medical studies. He became a student of medicine in 1905, qualifying M.B., Ch.B. in 1910. He held the appointment of house surgeon to the Whitehaven Infirmary, and became assistant to Dr J W Crerar at Maryport. He also held appointments at the West Cumberland Infirmary, and was in practice at St John's Troon, Ayrshire prior to joining the R.A.M.C. Thomas gained a commissioned at the rank of Captain in July 1915, and on the 30th September the same year, he entered the war in Malta as a regimental medical officer. During the summer of 1917, he was working in a general hospital at Alexandria, before being posted to the 1/2nd South Midland Mounted Brigade Field Ambulance. He was severely wounded through the chest, at Beit Ur El Tahta, while in the act of tending a wounded officer on 28th November 1917 and recovered sufficiently to be sent home. However he had a relapse whilst on the hospital ship and died at sea. A colleague wrote of his half-humorous, half-cynical, but always kindly, outlook on life. Thomas was the second son of the Rev Alexander McRae Craig M.A., and Margaret Craig.