In the 1805 campaign, Mathieu was named to command the 2nd Division of the VII Corps under Marshal Pierre Augereau. From 1806 to 1807 he served King Joseph Napoleon in the puppet Napoleonic Kingdom of Naples. In 1808 he was appointed to command the 1st Division of the III Corps in Spain. He fought at Tudela in November. In 1809 he transferred to Marshal Michel Ney's VI Corps and was present at Alba de Tormes. From 1811 to 1813 he commanded a division in the Army of Catalonia, leading his troops at Fort Monjuich, Montserrat, Altafulla, and Tarragona.