As aide-de-camp to Brigadier General Peyton C. March, Reinhart sailed for France on June 30, 1917, almost three months after the American entry into World War I. After arriving on the Western Front, the main theater of war, in late 1917, he assumed command of Battery 'A' of the 17th Field Artillery Regiment, part of the 2nd Infantry Division of the American Expeditionary Force (AEF), from February 12, 1918 to July 10, 1918 (in action during the defense of sector from March 21 to May 12, Battle of Bois de Belleau). Next, Reinhart commanded the 1st Battalion, 17th Field Artillery Regiment (Battle of Soissons, Ypres-Lys, and Meuse-Argonne). He commanded the battalion until the war ended on November 11, 1918 with the signing of the armistice with Germany. Reinhart was awarded the Army Distinguished Service Medal for his actions in combat during World War I.