According to the writings of Ennodius, bishop of Pavia in early 6th-century, Marolus was born on the banks of the Tigris in Mesopotamia. Probably due to the persecutions by Shapur II, Marolus moved before 380 to Syria where he grew up. He later moved to Rome, where he became friend of Pope Innocent I, and finally to Milan, where he became the bishop in 408.
Marolus was the bishop of Milan during an invasion of Italy by the Visigoths and he helped the victims of the invasion. He probably took in Milan from Antioch the relics of saints Babylas of Antioch and Romanus of Caesarea, and he founded a church in Milan known as Basilica Concilia Sanctorum or church of San Romano, today no more extant, near the present church of San Babila.