Grimminger joined the Nazi Party (NSDAP) in 1922 and became a member of the Sturmabteilung (SA). He took part in street-fighting in Coburg in 1922 and in the Munich Beer Hall Putsch of 8 November 1923. After serving in the Brown House, the general headquarters of the NSDAP, he was selected in 1926 to become a member of the Schutzstaffel (SS).