Józef Kustroń and was stationed in and around the towns of Nowy Sącz, Bielsko, Cieszyn, Bogumin, Kraków.

During the Invasion of Poland, the division was attached to Bielsko Operational Group of the Kraków Army.

Finally, the division managed to escape from German forces and cross the San river overnight of September 12 near Jarosław.

However, on September 16 the remnants of the 21st Mountain Division were finally surrounded and most of them either capitulated or were disbanded by their commanders.

The division's commanding officer, Józef Kustroń, was killed on the battlefield while personally leading his soldiers in an assault.