He volunteered for the Navy in 1804 and participated in the recapture of Fort Le Diamant on Martinique, then fought at Trafalgar as an aspirant on board Bucentaure, the admiral's flagship.

In 1822, as lieutenant de vaisseau, Alexandre was awarded the Légion d'honneur for successfully rescuing the crew of the frigate Africaine, shipwrecked on the Newfoundland coast.

He then commanded the frigate Iphigénie in the Mexico expedition participating in the blockade of Veracruz and bombardment of San Juan de Ulúa in 1838, then the occupation of Argentine island of Martin-Garcia in 1839.

Promoted to contre-amiral on 30 April 1840, Alexandre then served as major-general of the navy at Toulon, then Maritime Prefect at Cherbourg-en-Cotentin, before commanding the training squadron in the Mediterranean.

He then commanded France's Baltic fleet during the Crimean War, with which he bombarded the Russian fortress of Bomarsund and received its surrender on 16 August - as a reward he was promoted to admiral on the following 2 December.