Smyth volunteered at the outbreak of World War I even though he had been offered a position as Professor of Mathematics at the Royal School of Military Engineering, Chatham. He was sent to France on 17 August 1914 with the 14th Company and promoted to Captain in October. Serving throughout the war he was seriously injured on a number of occasions, losing his left arm at the elbow during the Battle of the Aisne at Givenchy whilst rescuing a wounded soldier who was caught in the open under heavy shellfire. From 1916 onwards he left the Royal Engineers and served with the Kings Own Scottish Borderers. He was mentioned in despatches seven times and awarded the DSO twice. He served with the 6th Battalion of the KOSB at the Battle of Arras, the only unit to attain its objective on 3 May 1917. He was cited for a mention in dispatches for "consistent skill and daring," after being severely wounded, receiving shrapnel pieces in his right shoulder which at the time was believed would permanently weaken his arm. The citation in the "London Gazette" of 18 July 1917 read as follows: