Son of Ferdinand Albert II (d. 1735) and Antoinette Amalie, dau of Louis Rudolph, Duke of Brunswick-Lüneburg. Brother of Prince Ferdinand. Married to Philippine Charlotte, dau of King Frederick William I of Prussia. Father of Karl Wilhelm Ferdinand and at least eight other children.
Karl was much interested in the development and welfare of his Dukedom and founded the Collegium Carolinum (which later became the Technical University of Brunswick) and the Fürstenberg Porcelain Company. Unfortunately, he seemed to be more enthusiastic than able, and so his son Karl Wilhelm Ferdinand took over government business in 1773, 7 years before the death of his father.
Boswell was presented to the Duke and Duchess at the court of Brunswick on June 27, 1764, and he enjoyed their hospitality several times during the next couple of months. Boswell also met the Duke in the royal gardens in Berlin on July 22, 1764; he subsequently wrote "How worthy and how amiable a prince is this! He made me very happy."