Ernst also had an older brother named Salentin, who fell in imperial military service on December 5, 1619.

At the beginning of 1633, as general of artillery, he led an advance in support of the Elector of Cologne, Ferdinand, against the Swedes.

When the Swedes under Baudissin took Hammerstein Castle in return and attacked Remagen, Isenburg moved towards them and pushed them back.

[4] In mid-March, however, he had to lift the siege of Andernach and retreat across the Rhine in the direction of Linz am Rhein before the Swedish relief army led by Baudissin's successor, Christian I, Count Palatine of Birkenfeld-Bischweiler.

On 29 September 1645 he was also appointed head of finances (grand treasurer) of the Netherlands, the tax administration of the king of Spain.