Before and during World War I, Leeb served as an adjutant in artillery units and then was appointed a General Staff officer.

In June 1917, he was transferred to the General staff in the XVth Royal Bavarian Reserve Corps and an infantry division.

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.

Later, his unit was transferred to Johannes Blaskowitz's 8th Army in its attack from the east-central region of Germany into west-central Poland, before sweeping on towards Warsaw.

His predecessor, Karl Becker, had committed suicide because he was unable to properly supply the field units with ammunition.