He was baptized Ferdinand Kozak[1] in an upper middle class family in Ljubljana, then part of the Austro-Hungarian Empire.
After the War, he studied Slavic philology in Prague, where he became friends with many Slovenes living in the Czechoslovak capital, such as the painter Božidar Jakac, philosopher Anton Trstenjak and sociologist Mihajlo Rostohar.
As a fierce opponent of Yugoslav centralism and nationalism, Kozak was among the founders of the left liberal journal Sodobnost, which he edited together with Josip Vidmar, Fran Albreht, and Stanko Leben.
After the Axis invasion of Yugoslavia in April 1941, Kozak was among the founders of the Liberation Front of the Slovenian People, a communist-dominated left-wing coalition that eventually fought the Nazi German and Fascist Italian annexation of Slovenia.
After the Italian armistice, he served as member of the Yugoslav Partisan war mission to the allies in Bari.