Today the Church celebrates the Feast of Saint Mark, known in the New Testament as John Mark. According to Acts he accompanied Paul and Barnabas on their missionary journey until he decided to leave. This angered Paul and was one of the reasons that Paul and Barnabas eventually split.
Although his name appears no where in the gospel, his name has been attached to the third gospel as its author. According to Papias, Mark accompanied Peter in Rome and wrote down Peter's account of Jesus' life into the gospel we now call Mark.
Later tradition records that he was the first bishop of Alexandria where he was eventually martyred.
For more information on Mark you can go here and here.