He became Duke of Württemberg-Juliusburg when his father died in 1684, although he stood under guardianship until he came of age in 1704.

On 20 December 1703 in Meiningen, he married Wilhelmine Louise (1686–1753), the daughter of Duke Bernhard I of Saxe-Meiningen.

In 1742, Karl asked his country's captain for a new wig, however, it was not granted due to an empty treasury.

He then sold off the Lordship of Goschütz and the city of Twardogóra, the latter to Count Henry of Reichenbach-Goschütz (1705–1775).

Since his marriage had been childless, the Duchy of Bernstadt fell to his cousin Charles Christian Erdmann, who was thus able to unite all the Silesian possessions of the House of Württemberg in a single hand.