On 17 May 1803, he succeeded as the Prince of Nassau-Usingen when his elder brother, Charles William, died without male heirs.

Since Friedrich August's sons had died young, it was also agreed that the combined duchy and sole rulership would pass to the heir of the Nassau-Weilburg line.

After Napoleon's defeat at the Battle of Leipzig, Friedrich August joined in the allies fighting against the French emperor.

Friedrich August was an enlightened and liberal ruler who established reforms such as the elimination of tax privileges for the nobility, the introduction of press freedom and the constitution for a modern state.

On 9 June 1775, Friedrich August married Princess Luise of Waldeck and Pyrmont (born 29 January 1751 in Arolsen, died 17 November 1816 in Frankfurt am Main), daughter of Karl August, Prince of Waldeck and Pyrmont and Countess Palatine Christiane Henriette of Zweibrücken-Birkenfeld.