[1] Jäger was born in a corner house in Eimsbüttel as the son of a shoemaker and began his career with the youth team of SC Union 03 Altona, for which he was also active as a schlagball.
He was signed by Hamburg's Altona 93 in 1907, making his debut on Christmas Day of 1907 against the Dutch club FC Dordrecht, and going on to score over 2000 goals in over 700 amateur matches.
Jäger also played 51 matches for the national team of the Northern German Football Association, including 26 appearances in the Bundespokal, where he scored 35 goals and won the title in 1914, 1917 and 1919.
[2] Unlike his powerful HSV counterpart Otto Harder, with whom he stormed together in the North German selection and also in the national team, Jäger repeatedly convinced the critics with intelligent passes, especially in the wings.
[3] Shortly before the end of World War II, Jäger died while attempting to defuse a bomb at the Elbe riverside in Hamburg[6] right after an American air raid,[7] and is buried in Altona Main Cemetery.