From what I have read, he was one of many WW1 veterans who were awarded Purple Hearts when he award was reinstated in the 1930s.
George S. Patton, Jr. sailed to France in 1917 and began studying tank tactics with the Allies. He established a tank school in Bourg, France, trained the first American tank crews and commanders, and led a 345-tank brigade into combat at Meuse-Argonne. He was severely wounded in the leg by gunfire on 26 September 1918 and, on account of that combat injury, was awarded the Purple Heart in 1932.