He was the son and successor of John III and brought Naples back into the Byzantine fold, receiving the title eminentissimus consul et dux, atque imperialis anthipatus patricius.
He devastated the surrounding countryside and took an enormous booty before an army of Otto I, Holy Roman Emperor forced the Greeks to retreat.
In 974, Marinus allied with Manso I of Amalfi and Landulf of Conza to depose Gisulf I of Salerno.
On 4 November 981, the Emperor Otto II was in Naples, probably with the permission of Marinus, who was moving away from his pro-Byzantine policy.
In 975, Marinus made a donation to the urban church of Saints Severinus and Sossus in Naples.