[2] From his parents' marriage, he had several siblings, including Alexandre de Forth-Rouen and Eliza Parker Forth (wife of the Hon.
After his father's death in April 1809, shortly before his youngest brother's birth, his mother married Baron Alexandre-Jean-Denis Rouen des Mallets,[3] the Mayor of Taverny, with whom she had another daughter, Louise Rouen des Mallets (who married Charles Louis, Viscount Terray de Morel-Vindé in 1839), Frederick's half-sister.
[4] He was an army officer and lieutenant-governor in the British West Indies for some six years, where he was sent to initiate a government on the separation of those islands from the Bahamas.
[7] He was a captain in the Scots Fusiliers and was appointed by Governor Sir George Arthur in the first visiting magistracy created in Tasmania.
He prepared the first code of standing regulations for the management of some thousands of European convicts employed upon the public works and roads of the colony.