After the war, he resumed his studies, and worked in Bolzano as an architect.
His first contact with film came in 1921, when he helped director Arnold Fanck on one of his mountain films. The main actor could not perform the stunts required, and so Trenker assumed the leading role. He gradually assumed more roles on the set, and by 1928 was directing, writing and starring in his own films. By now he had abandoned his job as an architect to concentrate on his films. He married Hilde Bleichert, with whom he had four children.
The main theme of Trenker's work was the idealization of peoples connection with their homeland and pointing out the decadence of city life (most clearly visible in his 1934 film "Der verlorene Sohn" / "The Prodigal Son"). This loosely played into the hands of Nazi propagandists, who seized upon the nationalistic elements of his work. However, Trenker refused to allow his work to be subverted as such and eventually moved to Rome to avoid further governmental pressure. This, though, was not to be and after a pair of documentary films Trenker returned to Bolzano and quit making movies. The style he had developed in the thirties was not limited to nationalistic, folkloristic and heroic clichès, however; his impersonation of a hungry, downridden immigrant in depression time New York was regarded as one of the seminal scenes for future Italian neorealism by the likes of Roberto Rossellini.
After the war Trenker was accused of fascist opportunism but eventually the charges were dropped. In the mid 1950s he again was able to make movies, though by 1965 he had switched mainly to the documentary form, focusing mainly upon the Austrian province of Tyrol.