Between 2010 and 2017 he served as Minister for Interior and Local Government of North Rhine-Westphalia, in the cabinet of Minister-President Hannelore Kraft.

After his high school diploma ("Abitur") in 1981 Jäger finished his vocational training as a merchant in wholesale and foreign trade.

In the negotiations to form a Grand Coalition of Chancellor Angela Merkel's Christian Democrats (CDU together with the Bavarian CSU) and the SPD following the 2013 German elections, Jäger was part of the SPD delegation in the working group on internal and legal affairs, led by Hans-Peter Friedrich and Thomas Oppermann.

The opposition parties in the Landtag criticised him for "acting too late",[3] CDU Secretary General Peter Tauber urged him to step down because of the events.

[4] Jäger was a SPD Party delegate to the Federal Convention for the purpose of electing the President of Germany in 2017.