Son of Karl I (1713-1780), Duke of Brunswick-Wolfenbüttel, and Philippine Charlotte of Prussia. Married (1764) to Princess Augusta of Hannover, sister of King George III of England. He was the de facto ruler of Brunswick-Lüneburg from 1773, formally succeeding his father in 1780. An able military commander he fought in several wars, serving as field marshal in the Prussian army against Holland in 1787 and fighting french revolutionaries in 1793. He died from wounds received in the battle against Napolean at Jena-Auerstedt in 1806.
Boswell went to see the arrival of Karl Wilhelm Ferdinand and his wife to Utrecht on February 10, 1764, shortly after their wedding. On June 5, 1764, he was invited to sup with the Duke, which, accordingly, he did, and on June 27 he was "politely received" by the Prince in Brunswick.