Mastalus I was the ruler of Amalfi from 900 to his death. He was the son of Manso I, who held the title of Prefect. In 900, he was elevated to co-rulership with his father. In 914, he became sole ruler, taking the title Patricius. During his lengthy rule, he co-ruled at times with his two sons, Leo and John, and his grandson, Mastalus. Upon his death, he was succeeded by his grandson, Mastalus II, who was raised to the status of dux (duke).