August Eigruber joined the NSDAP in 1927. In 1930, he became head of the Hitler Youth and party leader for the region of Steyr and, in 1931, leader for the city of Steyr. He became leader of the then illegal NSDAP for Upper Austria in 1936.
Eigruber was a particularly enthusiastic Nazi. He was on good terms with Hitler and Bormann and was successful in asserting his ideas about the rebuilding of Linz over the head of the official architect Roderich Fick.