Just over a year after the
British entry into the First World War, Carlin re-enlisted on 8 August 1915; the army refunded half (£9) of the money he had bought himself out with in 1909. Serving in Belgium with the 18th Royal Hussars, he was awarded the
Distinguished Conduct Medal on 5 August 1915, and was later commissioned as a
second lieutenant in September 1915. He was promoted to
lieutenant in May 1916. Carlin lost a leg in the
Battle of Longueval/Delville Wood, on the Somme in 1916, while commanding a
Royal Engineers Field Company section holding a trench against repeated German counter-attacks. For this action he was awarded the
Military Cross in October. Carlin joined the
Royal Flying Corps (RFC) in 1917, following his recovery. On 12 March 1918, Carlin was seconded from the Royal Engineers to the RFC. After serving as an instructor at the
Central Flying School, he was posted in May 1918 to
No. 74 Squadron RAF flying
S.E.5As, where he earned his nickname "Timbertoes". Carlin is recorded as an
ace balloon buster, with five balloons downed; he was also an ace against aircraft, with four machines claimed destroyed, and one aircraft 'driven down out of control'. His exploits earned him the
Distinguished Flying Cross. On 9 August 1918, Carlin was promoted to temporary captain. In early September he was involved in a mid-air collision with his commanding officer, Major
Keith Caldwell, but was relatively unscathed. On 21 September Carlin was shot down over
Hantay by
Unteroffizier Siegfried Westphal of
Jasta 29 and held as a
prisoner of war. He was repatriated on 13 December 1918 and admitted to the
RAF Central Hospital on Christmas Day 1918. Carlin relinquished his commission on "account of ill-health contracted on active service" on 7 August 1919, and retained the rank of lieutenant. ==Inter-war years==