Karl Anselm was the eldest son of Alexander Ferdinand, 3rd Prince of Thurn and Taxis and his first wife Margravine Sophie Christine of Brandenburg-Bayreuth.

[citation needed] Following the death of his first wife, Karl Anselm married that same year morganatically to Elisabeth Hildebrand.

During the invasion of French troops in the Austrian Netherlands in 1794, the local properties of the Thurn und Taxis family were seized.

Due to the Napoleonic Wars, Karl Anselm's Imperial Reichspost gradually lost more and more postal districts beginning with the Austrian Netherlands, thus depriving the post of important sources of revenue.

Only under his son and successor, Karl Alexander, was the Thurn and Taxis family able to re-establish its postal system as the private company Thurn-und-Taxis-Post.