With the death of his father on 13 November 1805, he became nominal Generalpostmeister of the Imperial Reichspost until the resignation of Francis II, Holy Roman Emperor.

Karl Alexander also worked for the Thurn and Taxis postal empire, operating during a decline due to the gradual loss of territory as a result of the Napoleonic Wars.

Karl Alexander and Therese had seven children: After the end of the Holy Roman Empire, the Thurn and Taxis postal system continued to survive as a private company.

After the Congress of Vienna, Karl Alexander took over the Hessian and Thuringian postal services, as well as those in the Hanseatic League cities of Bremen, Hamburg, and Lübeck, and Schaffhausen.

Karl Alexander also received as the family head of the House of Thurn and Taxis, Prussian possessions in the Grand Duchy of Poland.