[2] In the 14 September 1930 Reichstag election, Lindau was the only large town in southern Bavaria in which the Nazis became the largest party, with 22.1% of the vote.

[3] On 24 April 1932, Siebert was elected as a Nazi Party member of the Bavarian Landtag where he sat until its dissolution in October 1933.

[4] Also on 10 March, he was named Staatskommissar (State Commissioner) for the Bavarian Ministry of Finance in the administration of Franz Ritter von Epp, the Reichskommissar appointed by the central government.

On 12 November 1933, he was elected as a member of the Reichstag from electoral constituency 24 (Upper Bavaria and Swabia) and served until his death.

Furthermore, Siebert was hampered by his lack of command authority over the Party's paramilitary units in Bavaria, the SA and the SS, which were tightly controlled by Ernst Röhm (and his successors) and Heinrich Himmler, respectively.

[12] From 1933 until his death, Siebert was chairman of the Supervisory Board (Aufsichtsrat) of the Bayerische Berg-, Hütten- und Salzwerke AG (Bavarian Mining, Metallurgical and Salt Works).

In March 1939, Siebert was made president of the Deutsche Akademie, a German cultural institute and the precursor of the Goethe-Institut.

[14] Siebert's tenure saw an increasing politicization of the organization when it was officially placed under the auspices of the Reich Ministry of Public Enlightenment and Propaganda and its operations abroad were overseen by Reichsminister Joseph Goebbels.

[15] After the outbreak of the Second World War in September 1939, Siebert was made a member of the Reich Defense Committees for Wehrkreise (Military Districts) VII and XIII, which encompassed Bavaria.