Leeb entered Army service on 7 July 1901. He attended the War School in Munich, the Bavarian Artillery & Engineer School, and then the Bavarian War Academy. Before and during World War I, Leeb served as an adjutant in artillery units and then was appointed a General Staff officer. Leeb was promoted to captain on 1 June 1915. In June 1917, he was transferred to the General staff in the XVth Royal Bavarian Reserve Corps and an infantry division. Leeb participated in battles around Lorraine, Northern France, Galicia, the Carpathian Mountains, Flanders and the German withdrawal from Northern France. Leeb's older brother, Wilhelm Ritter von Leeb, had the knightly rank of Ritter and the nobiliary particle of von, not by birth, but thanks to the conferment of the Bavarian Military Order of Max Joseph and a patent of nobility. Hence, the older brother had von between his names, but the younger brother did not.