During Nazi Germany's occupation of Denmark, Frederik acted as regent on behalf of his father from 1942 until 1943.

During Frederik's reign, Danish society changed rapidly, the welfare state was expanded and, as a consequence of the booming economy of the 1960s, women entered the labour market.

[6] Prince Frederik was born on 11 March 1899 at his parents' country residence, the Sorgenfri Palace, located on the shores of the small river Mølleåen in Kongens Lyngby north of Copenhagen on the island of Zealand in Denmark, during the reign of his great-grandfather King Christian IX.

[citation needed] He was baptised in the Garden Room at Sorgenfri Palace on 9 April 1899 by the royal confessor Jakob Paulli.

[7] The young prince had 21 godparents: Christian IX of Denmark (his paternal great-grandfather); Crown Prince Frederik of Denmark (his paternal grandfather); the Dowager Grand Duchess Anastasia of Mecklenburg-Schwerin (his maternal grandmother); Grand Duke Michael Nikolaevich of Russia (his maternal great-grandfather); Dowager Grand Duchess Marie of Mecklenburg-Schwerin (his maternal step-great-grandmother); Prince Carl of Denmark (his paternal uncle); Princess Thyra of Denmark (his paternal aunt); Frederick Francis IV, Grand Duke of Mecklenburg-Schwerin (his maternal uncle); George I of Greece (his paternal great-uncle); Albert Edward, Prince of Wales (his paternal great-uncle by marriage); Ernest August, Duke of Cumberland (his paternal great-uncle by marriage); Grand Duke Alexander Mikhailovich of Russia (his maternal great-uncle); his first cousins once removed, Nicholas II of Russia, George, Duke of York, Prince George of Greece and Denmark and Georg Wilhelm, Hereditary Prince of Hanover; Crown Prince Constantine and Crown Princess Sophia of Greece (his first cousin once removed, and his wife); his paternal great-granduncles, Prince Johann of Schleswig-Holstein-Sonderburg-Glücksburg and King Oscar II of Sweden and Norway; and Crown Prince Gustaf and Crown Princess Victoria of Sweden (his first cousin twice removed and his wife).

[10] In the 1910s, Alexandrine considered the two youngest daughters of her cousin Tsar Nicholas II, Grand Duchesses Maria and Anastasia Nikolaevna of Russia, as possible wives for Frederik until the execution of the Romanov family in 1918.

They never wed.[11][12] Instead, on 15 March 1935, a few days after his 36th birthday, his engagement to Princess Ingrid of Sweden (1910–2000), a daughter of Crown Prince Gustaf Adolf (later King Gustaf VI Adolf of Sweden) and his first wife, Princess Margaret of Connaught, was announced.

[citation needed] Their daughters are: From 1942 until 1943, Frederik acted as regent on behalf of his father who was temporarily incapacitated after a fall from his horse in October 1942.

In 1948, one year into the king's reign, the Faroe Islands obtained home rule and became a self-governing country within the Danish Realm.

The King then lay in state for six days until his funeral, during which period the public could pay their last respects.

[20] At the Copenhagen Central Station, the coffin was placed in a special railway carriage for the rail journey to Roskilde.

On 20 April 1982, a statue of King Frederik IX dressed in the uniform of an admiral was unveiled by the Copenhagen harbour on the 35th anniversary of his accession to the throne in 1947 and in the tenth year after his death.

[22] The Crown Prince Frederik Range in Greenland was named after him when it was first mapped by Sir Martin Lindsay in 1934 during the British Trans-Greenland Expedition.

The dish is called "Kong Fiddes livret" (English: King Frederik's Favorite).

The dish is supposedly one that was regularly served to King Frederik IX's on his birthday at Gråsten Palace.