He was born at Hanau, the son of Prince William, later William II, Elector of Hesse, and Princess Augusta of Prussia, daughter of Frederick William II of Prussia. During the French occupation of Hesse-Kassel he stayed with his mother in Berlin. The relationship with his father was bad, because of his father's affair with Emilie Ortlöpp. Frederick was educated at Marburg and Leipzig. He became co-regent on 30 September 1831, and Prince-elector in 1847. Under influence of his minister Hans Daniel Ludwig Friedrich Hassenpflug he conducted a reactionary policy, which made him very unpopular. He was forced to give in to the demands of the March Revolution, but reinstated Hassenpflug in 1850 after the revolution had been crushed. In the Austro-Prussian War he chose the side of Austria. His capital Kassel was occupied by Prussia, and, as a consequence of his refusal to negotiate, he was transferred as a prisoner to Stettin on 23 June. Hessen-Kassel was annexed by Prussia in the same year. Frederick William never accepted the Prussian dominance over his territory. Even after the creation of the unified German Empire, he tried to regain his throne. He died at Prague in 1875. Because of his morganatic marriage, his sons were excluded from succession. He was succeeded, as titular Prince-elector of Hesse, by Prince Frederick William of Hesse, from the house of Hesse-Rumpenheim.
Children
Frederick William morganatically married on 26 June 1831 to Gertrude Falkenstein, daughter of Johann Gottfried Falkenstein and wife Magdalena Schulz. Gertrude Falkenstein was the former wife of LtKarl Michael Lehmann, whom his father William II made her Her Illustrious Highness Countess of Schaumburg in and whom he made Princess of Hanau and to Horowitz in. They had nine children, some born before marriage, who were also made princes of Hanau, and granted the style of Serene Highness in 1862. Subsequently, the Prince-elector divorced Gertrud.
Augusta Marie Gertrude, married 1849 Ferdinand Maximilian III Prince zu Isenburg-Büdingen in Wächtersbach.
Alexandrine, married 1851 Prince Felix zu Hohenlohe-Oehringen
Frederich Wilhelm, married morganatically twice: 1856 Auguste Birnbaum; and in 1875, Ludowika Gloede; their children were Counts von Schaumburg, but post-1918 descendants bear the title Prince/ss von Hanau.
Moritz, married morganatically 1875 Anne von Lossberg; no children
Wilhelm, married 1stly 1868 Princess Elisabeth, daughter of George William, Prince of Schaumburg-Lippe; 2ndly 1890 Countess Elisabeth zur Lippe-Weissenfeld ; no children
