Among his siblings were Johann Franz Heinrich Carl von Ostein, Austrian Ambassador to England and Russia.

[3] One of his ancestors was Johann Heinrich von Ostein, Prince-Bishop of Basel during the Thirty Years' War.

Having supported the losing side in the Seven Years' War, the cost of reparations bankrupted Mainz.

In addition, the new buildings of St. Peter's and the Jesuit church were built, as well as the Bassenheimer Hof on Diethmarkt.

Johann Friedrich Karl died on 4 June 1763 and was buried in Mainz Cathedral.