After Charles' father died in 1718, Charlotte Amalia acted as regent for both Charles and his younger brother William Henry II. In 1728, Charles inherited the counties of Nassau-Ottweiler, Nassau-Idstein and Nassau-Saarbrücken from his second cousin Frederick Louis. These counties were then added to his county of Nassau-Usingen.
In 1734, he was declared an adult by Emperor Charles VI. In 1735, he and William Henry II divided their inheritance. Charles received Usingen, Idstein, Wiesbaden and Lahr; William Henry II received Nassau-Saarbrücken and some smaller territories. He then moved his residence from Usingen in the Taunus to Schloss Biebrich in Biebrich and continued the progressive policies of his mother.
On 26 December 1734 Charles married Princess Christine Wilhelmine of Saxe-Eisenach (born: 1711), a daughter of Duke John William III. The marriage produced four children:
In the second, morganatic marriage, Charles married Magdalene Gross from Wiesbaden (born: 1712). From this marriage he also had four children:
Philippa Catherine von Biebrich (born: Idstein 17 May 1744; died: 17 July 1798), married at Biebrich June 1773 to Baron Karl Friedrich von Kruse (1738–1806)
Karl Philipp von Biebrich, Count von Weilnau (born: 25 March 1746; died: 15 August 1789 Wiesbaden). The Nassau Family Pact was executed in 1783, but he was not invited to be a signatory because of the morganatic marriage of his parents.
Sophie Christine (born: 20 June 1750; died: 16 November 1750)
Wilhelm Heinrich (born: 15 February 1755; died: 6 April 1755)
