Prominent among the clergy, Mark, the son of Priscus, was elected to the See of Rome just eighteen days after the death of St. Sylvester. A Roman, Mark was described as a man who was "filled with the love of God, the guardian of justice, a true friend of Christ."
Mark is credited with instituting the ancient custom of a pope being consecrated by the bishop of Ostia. He is also said to have issued an edict whereby the bishops of Ostia were granted the pallium by the pope. The pallium, a circular band of wool, is the symbol of apostolic power worn by the pope; when it was bestowed on an archbishop, it symbolized his participation in that power.
Mark's pontificate, although short, occurred during times of great change. The heretic Arius was dying, nevertheless his followers continued to grow in strength in the East, and as a result such great leaders of the orthodoxy as Athanasius of Alexandria were deposed. Emperor Constantine still reigned, and he was no less generous toward the new pope. He bestowed on Mark two more basilicas: one in the cemetery of Balbina on the Via Ardeatina; and the other, the church of St. Mark, originally named for the pope.
Mark died peacefully on October 7, 336, the same year in which he was elected. He was buried in a place of his own choosing, the cemetery of Balbina. http://www.catholicculture.org/culture/liturgicalyear/calendar/day.cfm?date=2014-10-07