After his mother's death, his father remarried, in 1810, to Alexandrine Charlotte de Rohan-Chabot (whose husband, Louis Alexandre, Duc de La Rochefoucauld, was killed during the September Massacres of the Reign of Terror).

Boniface de Castellane entered the French army on the day of the coronation of Napoléon I of France (December 2, 1804) as an enlisted soldier in the 5th Light Infantry Regiment.

When Napoléon returned to Germany in 1809, de Castellane followed and he fought in that campaign at Abensberg, Eckmühl, Ratisbon, Aspern-Essling and Wagram.

In October 1812 he was made aide-de-camp of general Narbonne and was present at Krasnoi and the crossing of the Beresina.

De Castellane was part of the French army sent to secure Belgium's new found independence.

Through her second marriage, he was posthumously a grandfather of Marie Dorothée Louise Valençay de Talleyrand-Périgord (1862–1948), who married Karl Egon IV, the Prince of Furstenberg (1852–1896) in 1881.

After his death, she married her much older cousin Antoine's son, Jean de Castellane (1868–1965),[16] in 1898.