His marriage as hereditary prince at the imperial court in Paris on 4 February 1808 to Princess Antoinette Murat, a niece of Napoleon I's brother-in-law, the French Marshal Joachim Murat who was then Grand Duke of Berg, constituted a union between extended family members of the previously warring French imperial and Prussian royal dynasties following Napoleonic victories on the European continent. Following the death of his first wife on 19 January 1847, Karl married Princess Katharina (1817–1893), daughter of Karl III Albrecht, last reigning Prince of Hohenlohe-Waldenburg-Schillingsfürst, and the widow since 1845 of Count Franz Erwin von Ingelheim, on 14 March 1848.