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. In February 1806 he was promoted to 2nd lieutenant in the 7th Dragoons. The same month he was transferred to the 24th Dragoons with whom he served in the Kingdom of Naples. In 1808 de Castellane followed general Georges Mouton into Spain as an "aide-de-camp". When Napoléon returned to Germany in 1809, de Castellane followed and he fought in that campaign at Abensberg, Eckmühl, Ratisbon, Aspern-Essling and Wagram.