From 1930 he worked for UFA and together with Luis Trenker made Berge in Flammen ("Mountains in Flames") (1931). He then experimented with other genres, for example the comedy Die Gräfin von Monte Cristo ("The Countess of Monte Cristo") (1932) with Brigitte Helm and Gustaf Gründgens, and in the same year the flying drama film F.P.1 antwortet nicht with Hans Albers, Peter Lorre, Paul Hartmann and Sybille Schmitz in the leads. His lavish science fiction film Gold, released in 1934, is among the most successful German language films of the genre. Karl Hartl also directed the popular criminal comedy Der Mann, der Sherlock Holmes war ("The Man Who Was Sherlock Holmes") (1937).
After the Anschluss in 1938, when Austria was annexed to Nazi Germany, Hartl became director of production for Wien-Film, the newly-created and indirectly stated-owned body through which UFA, and beyond them the National Socialist German government, controlled the Austrian film industry. In this role, which he retained until the end of the war, he seldom undertook work on individual films himself but was nevertheless involved at a senior level with some of the most significant films of the Nazi period.
After 1945 he resumed film-making. On 3 July 1947 he set up in Salzburg, with the support of the Creditanstalt, the film production company Neue Wiener Filmproduktionsgesellschaft. One of his most acclaimed films of this period was Der Engel mit der Posaune ("The Angel With The Trunpet") (1949), which brought together many Austrian stars: Paula Wessely, Attila and Paul Hörbiger, Oskar Werner and Maria Schell.