He was born in Copenhagen, moved to Denmark at the age of three, and grew up there. He attended the university in Bonn, and then began a military career. In 1843 he was third in line for the Danish throne after the King's son and brother, Prince Ferdinand. On 28 January 1844, Frederick married Grand Duchess Alexandra Nikolaevna of Russia at St Petersburg. Frederick had come to St Petersburg as a prospective bridegroom for her sister Olga, but fell in love with Alexandra instead on the first evening he spent with the family. Although Olga was the elder daughter and also found Frederick to be an engaging young man, she stepped aside in favour of her sister, and even chaperoned the couple when they wanted to spend time together. The emperor and empress then gave their permission for Alexandra and Frederick to be married. Alexandra became acutely ill with tuberculosis shortly before her wedding, and this complicated the pregnancy which soon followed. She was never well enough to travel to Hesse and take up her new position with her husband. They stayed in St. Petersburg, where her health rapidly declined. She went into labor prematurely, three months before the child was due, and gave birth to a son, Wilhelm. The infant died shortly after he was born, and Alexandra died later the same day. Her parents were devastated and their grief would last until the end of their lives. She was buried at the Peter and Paul Fortress in St. Petersburg. The son was buried in Rumpenheim, now a borough of Offenbach am Main, Germany. In 1849 Frederick William joined HMS Cleopatra to train as a midshipman. The Cleopatra was reassigned to Singapore to take the place of HMS Maeander. She arrived in Singapore from Devonport via Rio de Janeiro under Captain Massie on 14 September 1849 and left with HMS Reynard for Labuan and China on 10 October. The Singapore paper mistakenly described the Prince as the son of the Danish king but the king had no sons, he was an heir to the throne. On 26 May 1853, Frederick married Alexandra's first cousin, Princess Anna of Prussia, at Charlottenburg Palace in Berlin. Although they had six children together, Frederick and Anna were never emotionally close, and it is speculated that one reason was because Fritz was unable to overcome his grief for his first wife.
He is important dynastically as a candidate for both the headship of the Hesse-Kassel dynasty and for the Danish throne. When Frederick William, deposed Elector of Hesse died in 1875, his sons were excluded from succession, because of his morganatic marriage. Therefore, Frederick succeeded him as titular Elector of Hesse. Frederick William died on 14 October 1884 at Hamburg.
Honours and awards
Friedrich Wilhelm received the following awards: ;German decorations ;Foreign decorations
