Siebert was born in Ludwigshafen, in the then Bavarian region of Palatinate, in 1874. He studied law, and after this became a civil servant in the Kingdom of Bavaria. He served as a prosecutor in Neustadt an der Weinstraße, and later in Fürth. Siebert was mayor of Rothenburg ob der Tauber from 1908 to 1919. He afterwards, in 1919, became the mayor of Lindau and while serving in this position in 1931, joined the National Socialist German Workers' Party from the Bavarian People's Party.

He became prime minister of Bavaria in 1933 with the Nazi takeover of power in Germany. Bavaria however, did not exist any more in its previous fashion as a Free State. Instead it was subdivided in Reichsgaue. As the prime minister of Bavaria, Siebert did not have the power and authority his predecessors had in the Weimar Republic. In this position, he was engaged in a power struggle with the Reichstatthalter of Bavaria, Franz Ritter von Epp, which he won.

He also held the posts of Minister of Finances (1933-1942) and for Economy (1933-1934/1936-1942).

He initiated the so called "Siebert Program" to fight unemployment in Bavaria. The program turned out to be insufficient to create new employment due to lack of funds within the Bavarian government and support from the German government.

Siebert also had personal orders from Hitler to look after the restoration of all castles in Germany and was especially interested in the restoration of Rothenburg from 1937 to 1941.

From 1933 until his death, he was chairman of the board of the Bayerische Berg-, Hütten- und Salzwerke AG (BHS), a large Bavarian mining company. He also served from 1939 to 1942 as the head of the Deutsche Akademie, a predecessor of the Goethe Institut.

His brother was a highly decorated officer in the Wehrmacht, the Generalmajor Friedrich Siebert (1888–1950)