During the 1930s, he studied stage directing and acting in Leningrad and Moscow, spending two years as an actor in the Ural Mountains.
Kostetskyj began his literary career at this time authoring Russian-language reviews of theatrical performances; his first publication, signed with his pen name incorporating his mother's maiden name, was written in Vinnytsia in 1941.
In the early 1940s, following the outbreak of the Second World War, he returned to the German-occupied Vinnytsia and lived there until the autumn of 1942, when he was deported to Germany for forced labor.
Kostetskyj continued an active literary career in the displaced persons camps after the war in West Germany, continuing to write and publish works in a combined traditional and modernist style, briefly publishing a short-lived artistic and literary journal, and becoming one of the founders of the Artistic Ukrainian Movement (MUR, Ukrainian: Мисте́цький Украї́нський Рух).
From 1949 to 1969, Kostetskyj was the editor of Ukraine and the World (Ukrainian: Україна і Світ, Ukraïna i svit), a journal of cultural, literary, scholarly, and political affairs published by Ilia Sapiha in Hannover, West Germany.