Mainville-Joseph Soulouque is the son of Prince Jean-Joseph, Duke of Port-de-Paix, and the eldest child of his father's second marriage to Princess Célestine Dessalines, daughter of Emperor Jacques I.
In 1849, when his uncle became Emperor of Haiti, he obtained the title of Duke of Fort-Royal and Prince of the Blood, as the sovereign's first nephew.
As Faustin had just lost his one and only son, he then thought of adopting his nephew, Mainville, and making him his new heir.
Finally, he modified the rules of succession, to allow his eldest daughter, Princess Olive, to succeed him.
As a prince of the blood, Mainville received the title of imperial highness on August 26, 1849, the date of the proclamation of the Second Empire and his uncle's accession to the throne.