Born on 14 March 1891 in Rzeszów, Józef Zając studied at the Jagiellonian University in Kraków, graduating with a PhD in philosophy in 1915.

In 1914 he joined the Polish Legions in First World War; he commanded the 3rd, 6th, 5th and again 3rd regiments (pułks).

In 1918 he joined the Blue Army of general Józef Haller; until mid-1920 he spent most of his time in France (among other things, he attended the École Supérieure de Guerre).

In February 1939 he succeeded General Ludomił Rayski as the Commander of Polish Air Force.

Next year he became the commander of the Polish Air Force in the Middle Eastern theatre and was promoted to division general.