During World War II, he collaborated with the Communist-led Liberation Front of the Slovenian People, which led him to some conflict with the collaborationist bishop of Ljubljana, Gregorij Rožman.

He is best known for his patriotic historical novel Under the Sun of Freedom [sl] (Slovene: Pod svobodnim soncem), written between 1906 and 1907 and partially modelled on Quo Vadis.

The narrative was set during the late Migration Era of the 6th century AD, and depicted a three-way conflict between the primitive but vital South Slavic tribes, the advanced but decadent Byzantine Empire, and the brutal and culturally-alien Avars.

The central character is the young Slavic prince Iztok, who makes his way incognito to Constantinople and joins the army in order to learn Roman military tactics, but also falls in love with a Greek noblewoman, navigates Byzantine court intrigue, and converts to Christianity.

Finžgar also wrote short stories and tales for children, the best known of which is Mr. Squall (Gospod Hudournik), a set interlinked humorous adventures of a city-dweller who delights in holidaying in the high Karawanks.