After college, Mattos played for the San Diego Chargers. He would earn a spot on the Chargers roster in 2003 with his impressive performances in training camp, despite having not even been initially recruited by a major college. He would last two years with the Chargers, and four years in the League overall, also being signed by the Denver Broncos, and later the Tennessee Titans. Knee problems would limit his effectiveness and chances of game action in later years.