Little is known of the early life of Bp. Mark. His date of birth is unknown, and he is said to have been a Roman. His father's name was Priscus. Mark apparently was a member of the Roman [[clergy]], either a [[deacon]] or a [[priest]]. After the repose of Bp. [[Sylvester I of Rome|Sylvester I]], Mark was [[consecration of bishop|consecrated]] the bishop of Rome on [[January 18]], 336.
During his episcopate, Bp. Mark invested the Bishop of Ostia with the [[pallium]] and confirmed the power of this bishop to consecrate newly elected bishops of Rome. Bp. Mark was also credited with building two churches, the [[basilica]] of San Marco in Rome and a cemetery [[church]] outside the city over the Catacomb of Balbina.