During the early years of the regime, he was a leader in the efforts to unify the German Protestant churches under the Nazi leadership principle.

From 1914 to 1918, he served in the Imperial German Army and took part in the First World War, attaining the rank of Oberleutnant and earning the Iron Cross, 2nd class.

Following the Nazi seizure of power, Jäger was appointed a Ministerialdirektor and head of the church department in the Prussian Ministry of Science, Culture and Public Education in May 1933.

[5] On 11 April 1934, Jäger was appointed legal advisor to the German Evangelical Church (DEK) by Reich Bishop Ludwig Müller.

[7] Following the outbreak of the Second World War and the invasion of Poland in September 1939, Jäger was appointed as the deputy to Arthur Greiser, the Chief of Civil Administration in the Warthegau, an area that was annexed to Germany.

On 26 October 1939, Jäger was named Regierungspräsident of the newly-established Regierungsbezirk (government district) of Posen and the deputy to Greiser, who had meanwhile risen to the position of Reichsstatthalter (Reich Governor).

[8] Priests, monks and other church officials were arrested, deported to the General Government, transported to Nazi concentration camps in Germany or shot.