Today is the feast of St. Mark the Evangelist, one of the so-called "red days", a reference to the liturgical color of the day that is used to note the feast of a martyr.
In the Epistle to the Colossians, chapter 4, he is identified as a member of Barnabas' family.
In the Acts of the Apostles, he figures often as the disciples meet at the house of Mark's mother, Mary; Paul and Barnabas would bring Mark on a missionary journey with them, their first such journey, but Mark would leave prematurely for reasons that are not identified and return to Jerusalem. On Paul and Barnabas' second missionary journey, Paul took Silas instead of Mark, as he felt that Mark was not reliable, so Barnabas made a separate journey with Mark as his partner.
Paul would, though, refer to Mark in both Colossians and the First Letter to Timothy as a person of trust, so perhaps whatever controversy existed between Paul and Mark had been reconciled by that time.
Outside of scripture, Papias' 2nd century work Origins of the Gospels refers to Mark as Peter's aide and interpreter. It is possible that Mark was in Rome with Peter at the time of the latter's execution. According to pious legend, Mark then moved to Alexandria, Egypt where he would meet his own martyrdom.
We recognize him today primarily as the composer of the Gospel that bears his name; the oldest Gospel in the New Testament. The lections for his feast day may be found here.
Almighty God, by the hand of Mark the evangelist you have given to your Church the Gospel of Jesus Christ the Son of God: We thank you for this witness, and pray that we may be firmly grounded in its truth; through Jesus Christ our Lord, who lives and reigns with you and the Holy Spirit, one God, for ever and ever. Amen.