Prince Frederick of Hesse-Kassel

His father, the then hereditary prince (who reigned from 1760 and died in 1785) had in 1747 left the family and soon converted Catholicism, and in 1755 formally ended his marriage. The young prince Frederick, together with his two elder brothers, were with their mother the Landgravine Mary and became fostered by Protestant relatives in 1747. Soon the family moved to Denmark to be guests of her sister Louise of Great Britain, who died in 1751. His two elder brothers married Danish princesses - their first cousins - in 1763 and 1766 respectively. They remained in Denmark, becoming important lords and royal functionaries. Only his eldest brother returned to Kassel, in 1785 when ascending the landgraviate. In 1815, the prince was in command of the Royal Danish Auxiliary Corps mobilized as part of the Seventh Coalition against Napoleonic France.

On 2 December 1786 in Biebrich, he married Princess Caroline of Nassau-Usingen (4 April 1762 Biebrich - 17 August 1823 Offenbach), a remarkable heiress of a family which became extinct in male line. 1781 he bought the castle of Rumpenheim, Offenbach, from his brother Carl, and it became the family's seat. He became known as Landgrave of Hesse-Kassel-Rumpenheim,[1][2][3] and his descendants are known as the Hesse-Kassel-Rumpenheim branch of the House of Hesse, one of only two branches that survived to the present day.

• William (24 December 1787 – 5 September 1867), married Louise Charlotte of Denmark (1789–1864) and was the father of Louise of Hesse-Kassel (wife of Christian IX of Denmark).
• Karl Friedrich (9 March 1789 – 10 September 1802)
• Friedrich Wilhelm (24 April 1790 – 25 October 1876)
• Ludwig Karl (12 November 1791 – 12 May 1800)
• Georg Karl (14 January 1793 – 4 March 1881)
• Luise Karoline Marie Friederike (9 April 1794 – 16 March 1881), married Graf George von der Decken (1787–1859), King's Hanoverian General of the Cavalry (Germany)
• Marie Wilhelmine Friederike (21 January 1796 – 30 December 1880), married Georg, Grand Duke of Mecklenburg-Strelitz (1779–1860)
• Augusta Wilhelmine Luise (25 July 1797 – 6 April 1889), married Prince Adolphus, Duke of Cambridge (1774–1850) and was the maternal grandmother of Mary of Teck (wife of George V of the United Kingdom).