After training with the French Army, the 5th Division was deemed ready for combat and was sent to occupy French trenches in the Vosges Mountains on the first of June. Gaines reported that the division fought in “minor engagements” during this time. However, the division’s first major action took place during the Battle of St. Mihiel in which fourteen American divisions and four French divisions were tasked with breaking the German salient in the region. The attack began on September 12, 1918 and progressed so rapidly that by September 13, all of the first week’s objectives had been met. Gaines’ 5th Division fought so well at St. Mihiel that German soldiers referred to them as “Die rote teufel”, which means “Red Devils.” Fighting did not cease entirely until September 15, the day when Gaines was hit in the shoulder by German rifle fire.
Gaines spent the rest of the war in hospitals in France, and returned home to the United States on November 23, 1918. He permanently lost the use of his left shoulder as a result of his wound. Reflecting on his wartime service, Gaines wrote that the war was “good fun until you were hit.” According to his daughter, Marion Gaines Cushman, her father's injury caused him pain for the rest of his life and at times would become re-infected.Leonard Gaines recovered from his wound and returned to the United States. After recovering, Gaines found work as a land appraiser. He married Anna Marion Brand, and raised three children.