Franz Joseph was born in Heiligendamm in the Grand Duchy of Mecklenburg-Schwerin,[citation needed] the second son of Wilhelm, Prince of Hohenzollern and Princess Maria Teresa of Bourbon-Two Sicilies.

During World War I Franz Joseph served in Germany's Imperial Navy (Kaiserliche Marine) as the second torpedo officer on the light cruiser SMS Emden at the Battle of Cocos.

[7] As a leading Roman Catholic nobleman and a near relative of the Habsburg, Bourbon, and Saxon dynasties, Franz Joseph did much to lend respectability to the Nazi Party.

[12] In a letter to Heinrich Himmler dated 3 January 1945, Franz Joseph proclaimed his continued devotion to the Nazi cause and unsuccessfully begged to be readmitted to the SS.

[citation needed] His twin brother was married to Maria Alix's sister, Princess Margarete Karola of Saxony (1900–1962).