 He was born in Bonn, the scion of an illustrious family of "Court Jews" () who had served as advisers and moneylenders to the Prince-Archbishops of Cologne in the Rhineland area for several generations.In 1789, at the age of 17, Oppenheim Jr. set up a small commissions and exchange house in Bonn, then the residence of Prince-Archbishop Maximilian Francis of Austria.Nine years later, after French troops had occupied the left banks of the Rhine, Oppenheim Jr. moved to the city of Cologne.