In 1907 Kossak was arrested by the Imperial Russian authorities for protesting against the foreign occupation of Poland. During World War I, in 1917, he joined the Polish Corps of Józef Dowbor-Muśnicki fighting for Polish independence. Kossak was the author of a book of memoirs called "Jak to było w armii austriackiej" (How it was in the Austrian Army) published in 1927.
He was a father of a novelist Zofia Kossak-Szczucka (1889–1968), and a twin brother of a renowned Polish historical painter Wojciech Kossak.
During the interwar period, his estate in Górki was a magnet for Polish intellectuals, among them, January Parandowski, Maria Dąbrowska, January Sztaudynger and Melchior Wańkowicz.