Leonard E. Rea was born on March 14, 1897, in Auburn in New York, and, after graduation from high school, decided to enlist in the United States Marine Corps on April 19, 1917. Following basic training, Rea was assigned to 17th Company, 1st Battalion, 5th Marine Regiment, stationed at the Philadelphia Navy Yard. He left for France almost immediately after regiment's activation and arrived in Saint-Nazaire in July 1917. Rea was promoted to the rank of corporal and decorated with the Marine Corps Good Conduct Medal for his enlisted service. He was sent to the trenches of the Toulon Sector near Verdun in March 1918 and subsequently participated in the Battles of Belleau Wood and Soissons.