Born in Steyr, Austria, after finishing middle school, Eigruber underwent training in geodesy and fine mechanics at the Austrian Federal Teaching Institution for Iron- and Steelworking. Thereafter, he was active in his profession. In November 1922 he joined the National Socialist Worker Youth of Austria, whose leader he became in 1925. In April 1928, he joined the Nazi Party, whose Steyr-Land district leadership he took up in October 1930. For his activities in the NSDAP, which was banned in Austria, Eigruber was sentenced to several months in prison.