From 1804, he lived in his family's original home in Denmark-Norway, where he made a career as an officer of the Danish army during the Napoleonic Wars.

[3] He was the third and youngest child and only son of Friedrich Karl Ludwig, Duke of Schleswig-Holstein-Sonderburg-Beck (1757–1816) and his wife, Countess Friederike of Schlieben (1757–1827).

[3] At the same time, the thirteen year old Prince Friedrich Wilhelm was sent to the Knight academy in Brandenburg an der Havel where he received his education from 1798 to 1802, after which he studied at the University of Leipzig from 1803 under the supervision of the father.

From 1805 to 1807, he served there as second rittmeiser in the Life Regiment Dragoons, where he took part in guarding the border and several times had the opportunity to demonstrate good military skills, especially in reconnaissance.

[8] However, after the British bombardment of Copenhagen in 1807, Denmark allied with France and the army was transferred to Zealand, just as Friedrich Wilhelm also returned to service in the capital.

Friedrich Wilhelm and Louise Caroline were engaged in November 1809, and were married on 26 January 1810 in the chapel of Gottorp Castle.

His grandchildren include among others Frederik VIII of Denmark, Queen Alexandra of the United Kingdom, George I of Greece, Empress Maria Feodorovna of Russia, Crown Princess Thyra of Hanover, Duchess of Cumberland and Teviotdale, and Friedrich Ferdinand, Duke of Schleswig-Holstein.