 Charles was born in Kassel on 19 December 1744 as the second surviving son of Hesse-Kassel's then hereditary prince, the future Frederick II, Landgrave of Hesse-Kassel and his first wife Princess Mary of Great Britain.His mother was a daughter of King George II of Great Britain and Princess Caroline of Brandenburg-Ansbach and a sister of Queen Louise of Denmark.His father, the future landgrave (who reigned from 1760 and died in 1785), left the family in 1747 and converted to Catholicism in 1749.