Thomas was educated at Stonyhurst, and at Queen’s University, Ireland, graduating M.D., M.S. in 1873. In his younger days he was a keen sportsman, with recreations including racquets, billiards, cricket, polo, shooting and racing. On 31st March 1874 he took up a commission within the Medical Services at the rank of Surgeon. In 1879 he saw active service in the Afghan War, being present at the capture of Ali Musjid and the expedition into Bayar Valley. For his service there he received the Afghan medal with clasp. In 1882 he saw active service again in the Egyptian War, being present in the battles of Kassassin and Tel-el-Kebir. For his service there he received the Egyptian medal with clasp and the Khedive’s Bronze Star. From 1884 to1885 he took part on the Soudan Expedition, being in charge of the Gubat Field Hospital, as well as moveable field hospitals. He also took part in the engagements in the return of the desert force to Korti. For his service here he received the Sudan Medal with clasp; was Mentioned in Despatches; and received special promotion to Surgeon-Major on 15th June 1885. On 25th March 1892 he was seconded for service with the Egyptian Army, taking part in the Nile Expedition. He gained promotion to the rank of Lieutenant-Colonel on 30th September 1893. In 1896 he took part in the Dongola Expedition as Principal Medical Officer, serving under the rank of Brigadier Surgeon Lieutenant Colonel from 9th July 1896. For his services here he was Mentioned in Despatches, received the British medal and Khedive’s medal with two clasps, as well as being appointed Companion of the order of the Bath. He then returned to Sudan from 1897 to 1898, taking part in the Nile Expedition as Principal Medical Officer of the Egyptian Army, being present at the battles of Hafir, Atbara, and Khartum. For his services here he was Mentioned in Despatches twice (24th May and 30th September 1898), received the Egyptian medal with two clasps, and was specially promoted to Colonel on 16th November 1898. From 1899 to 1900 he took part in the South Africa war, seeing active service again, this time as Principal Medical Officer, Infantry Division, in actions of Colenso, Spion Kop, Vaal Krantz, Tugela Heights, Pieter’s Hill, and the relief of Lady-smith. He was then given a temporary rank of Surgeon General whilst Principal Medical Officer, Natal on 5th November 1899, and took part in operations at Laing’s Nek, and in Transvaal. For his services here he was Mentioned in Despatches by Sir Redvers Buller on 30th March and 3rd November 1900; received the Queen’s South African Medal with six clasps, and was appointed Knight Commander of of the Order of St Michael and St George. On 1st June 1902 he gained promotion to the rank of Surgeon General, and served as Principal Medical Officer of His Majesty’s Forces in India until 1906. On 15th March 1911 he retired. At the outbreak of war, he was re-employed as Deputy Director of Medical Services in the Aldershot Command , then in 1916 as Director of Medical Service, Home Services. In 1921 he was appointed Colonel Commandant of the R.A.M.C. 
 Thomas died in London after a long illness. He was the 4th son of Henry Gallwey J.P. of Tramore, Co. Waterford; and the husband of Maud Margaret Howard, daughter of Captain J W J Gifford, 12th Lancers - married in 1901.