St. Mark the Evangelist, writer of the Gospel of St. Mark, can be found in multiple places in the Bible. Though he is never mentioned in the gospels themselves, he does pop-up on occasion in the New Testament.
First of all, St. Mark is often identified with the "John Mark" in the Acts of the Apostles.
Secondly, St. Mark appears in St. Paul's letters, such as this mention in his letter to Timothy.
More importantly, St. Mark is believed to be the Mark mentioned by St. Peter in his letter.
The Catholic Encyclopedia explains how an early tradition places St. Mark as a disciple of St. Peter and that his Gospel was derived from his discussions with him.
Modern biblical scholars tend to disagree on the identity of Mark in the Bible, while early Church tradition combines all the appearances of "Mark" in the New Testament and connects them to the Gospel writer.
Even though St. Mark wasn't numbered among the 12 apostles, he may have had a first-hand witness, St. Peter, to consult when he wrote his Gospel.
