FitzClarence was commissioned as an officer in the British Army in 1814. While a captain in the Coldstream Guards, FitzClarence commanded a small detachment of Guards to act in support of the police with the arrest of the Cato Street conspirators in 1820. The arrest was not straightforward, and a scuffle ensued.
The Naval and Military Gazette (May 1845) identified Sgt James Graham as the man who saved FitzClarence"s life.
Frederick FitzClarence gained the rank of Colonel in the service of the 36th (Herefordshire) Regiment of Foot. On 24 May 1831 he was granted the rank of a marquess" younger son.
He died in office in October 1854. They had two children:
Augusta FitzClarence (December 1824 – 18 October 1865)
William FitzClarence (b & d 1827).