In February 1918, during World War I, Wainwright was ordered to France. In June, he became assistant chief of staff of the U.S. 82nd Infantry Division, with which he took part in the Saint Mihiel and Meuse-Argonne Offensives. As a temporary lieutenant colonel, he was assigned to occupation duty in Germany with the 3rd Army at Koblenz, Germany, from October 1918 until 1920. Having reverted to the rank of captain, he was then promoted to major.