Catlin graduated from the Army War College one month after the outbreak of World War I. Subsequently, he was placed in charge of the Marine overseas training camp at Quantico, Virginia. In October 1917, Catlin was sent to France as commanding officer of the 6th Marines. From June 1–6, 1918, the 6th Regiment saw action in the front lines from Paris-Metz Road through Lucy-le-Bocage to Hill 142. On June 6, 1918, with his regiment attacking Bois de Belleau, he was wounded in the chest by a sniper and evacuated to a hospital the next day. It was the first time he had been wounded in 28 years of active service.