Maurin led a cavalry brigade in Andoche Junot's army during the 1807 Invasion of Portugal. He was named Baron of the Empire on 17 March 1808. He reported to François Étienne de Kellermann who commanded a cavalry division that included one squadron each of the 26th Chasseurs à Cheval, 1st, 3rd, 4th, 5th, 9th, and 15th Dragoons. He was appointed governor of Algarve province in the south of Portugal. When the revolt against the French occupation broke out, he was in his sickbed in Faro. He and 70 other French soldiers were captured by Portuguese partisans on 16 June 1808 and handed over to the captain of a British vessel as prisoners.