Thomas qualified L.R.C.P.I.&S.I. & L.M. in 1901, and D.P.H. in 1904 at the Royal College of Surgeons, Dublin. In 1902 he served as a Civil Surgeon abroad in South Africa. He joined the Army Medical Services at the rank of Lieutenant on 31st August 1903, then served in India from 1905 to 1909. During this time he received his promotion to Captain on 28th February 1907. On 28th March 1915 he gained the rank of Major, and on 25th April 1915 arrived at Aldershot from the training center at Llandrindod Wells to take up the position of the Officer Commanding the 9th Cavalry Field Ambulance. He continued this role until 26th April 1916, when he handed command of the unit over to Captain R B Price, and then proceeded to the 1st Field Ambulance for duty. He remained with the 1st Field Ambulance as the commanding officer until 29th October 1917, when he returned to England on leave. During this time he was granted the rank of temporary Lieutenant Colonel from 28th May 1916 to 30th October 1917. On 2nd November 1917 it was reported that he would be struck off the strength of the unit with effect 30th November 1917. Thomas then served in Mesopotamia from 1917 to 1920. From 28th August 1919 to 30th May 1920 he served under the rank of acting Lieutenant Colonel. In 1920 he served in India, and was in someway invalided. On 10th October 1923, he was granted the full rank of Lieutenant Colonel and was placed on retirement with pay. He ceased being on the Roll of Officers on 24th June 1935. He died at Brighton, Sussex.