He attended high school in Tolmin and Gorizia.
In the late 1920s, when his native region was part of Italy, Kosmač joined the militant anti-fascist organization TIGR. In 1930, he was arrested by the Italian Fascist authorities, but released the next year. He fled to the Kingdom of Yugoslavia, and settled in Ljubljana. In 1938, he was granted a scholarship granted by the French government, and he moved to Paris, where he worked for the Yugoslav embassy.
In 1940, he fled to London, where he worked at the British Broadcasting Corporation World Service.
In 1943, he went to Cairo, and in 1944 to Nazi-occupied Yugoslavia, where he joined the Yugoslav partisan resistance. After World World War II, he worked as a reporter and a screenplayer for the emerging Slovenian film production industry.
Among other, he wrote the screenplay for the film On Our Own Land. In 1956, he settled in the coastal resort town of Portorož, where he spent the rest of his life.
He died in Ljubljana, and was buried in his native village.
Slovenian Academy of Sciences and Arts]
In 1961, he became a member of the Slovenian Academy of Sciences and Arts.