He was a soldato in the Clemente Crime Family who ratted out Alberto Clemente and his organization to the authorities. He appears in the mission Witness - Frozen Memories. When Vito Scaletta is arrested for selling stolen gas stamps, Joe Barbaro is sent to find out who reported Vito. Through a series of events Joe learns that Mazzeo has turned "rat" and is under protection by the Empire Bay Police Department.
After following two detectives to a secluded location, Joe bursts through the door of the Yacht Club where Mazzeo was being held. He attempts to escape, but in the end he's caught and killed by Joe.