Who he was
Marinus, son of a priest named Palumbo, was born at Gallese and given to the service of the Roman Church at twelve. He went three times to Constantinople as a papal envoy, once as one of the legates at the Eighth General Council in 869, and on another mission was imprisoned for carrying out his orders firmly. John VIII made him Bishop of Cerveteri, treasurer and archdeacon; though already a bishop, he was probably elected on the very day John died.
What he is remembered for
Little is known of his short reign. He allowed Bishop Formosus of Porto back to Rome, held to his predecessor's condemnation of Photius, and at King Alfred's request freed the English quarter in Rome from taxes. Because some chroniclers confused his name with Martin, the later Pope Martin IV took that number.
Sources
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