The family, including St Aloüarn's father, had a history of service in the French Navy, and St Aloüarn joined the "Gardes de la Marine" in 1754. As a naval cadet, he joined his uncle, René de Rosmadec St Aloüarn, on the 74-gun warship "Espérance". In November 1755, as it returned from a campaign off Canada, "Espérance" was attacked and captured by and . St Aloüarn and his uncle became prisoners of war and were held in England for two years, before they were returned to France. Because of his bravery under fire, St Aloüarn was promoted to ensign.