Henry qualified B.A., M.B., B.Ch., B.O.A. at Trinity College, Dublin in 1900. He entered the R.A.M.C. at the rank of Lieutenant on the 27th June 1901, and served in India 1902-1908. On the 19th May 1905 he was promoted to Captain, then served in West Africa from 1909-1910 and 1911-1912. He was then promoted again on the 27th June 1913 to the rank of Major. Henry was stationed at Netheravon when war was declared. He entered the war in France on the 12th August 1914 as the Regimental Medical Officer for the 2nd Bn. Royal Irish Regiment, but was taken prisoner during the Battle of Mons. It was reported in the Lancet on 17th October 1914 that he had previously been reported missing but was now unofficially reported as a prisoner or war. On 5th September 1914, just as the retreat from Mons had ended, he was sent to Germany as a POW with Lt E S B Hamilton, Capt H M J Perry and Capt J H Graham - all of the R.A.M.C. In 1916, Henry became the Commanding Officer of the 139th Field Ambulance. He served under the rank of temporary Lieutenant Colonel from the 6th January to 20 November 1916. He retired on reduced pay on the 16th November 1920, although granted full rank of Lieutenant Colonel. He ceased being on the Roll of Officers on the 14th October 1928.