In the case of Mark as with Luke, he was not also included in the original twelve choosen by the Savior. We do not know the precise relationship with the Savior, although Mark may have been among the larger circle of the Saviors' early followers. Tradition says that Mark was a missionary companion of Peter and that the Gospel of Mark represents in large measure Mark's account of Peter's recollections of the Lord and his ministry. That seems probable. It seems so if you would listen to not only Elder McConkie but those who subscribe to the learnings of scriptures.
Again, Luke is identified as the beloved physician and companion of Paul and was thus a bit more distant from the historical Jesus than was Mark. Mark, is considered by many to have been very careful, a historian who gathered all the information he could from "eyewitnesses, and ministers of the word" (Luke 1:2) as he wrote his two-part work, Luke and Acts, which he dedicated to Theophilus, a person otherwise unknown to us.
According to many, Mark is traditionally said to have written his gospel in Rome during the time of Nero's persecution of the Church. Many of the slums of Rome had been burned, an act that is probably attributable to Nero, who wanted to make room for public buildings. Nero needed a scapegoat, however, and the Christians, who were not popular anyway because of their rather "narrow" religious views, were a group that could be easily blamed. Thus, the members of the Church among whom Mark worked and served were undergoing tremendous persecution, and suffering greatly. Apparently Mark longed to bring a message of hope to these people in the midst of their sufferings, as well as to explain through Jesus' own words and actions why people suffer in this world when they are doing what the Lord requires of them. Thus, the persecution that Peter and Mark faced in the Roman church determined to a large degree Mark's focus on the suffering in Jesus' life and the lives of his followers. Yet, I do believe Mark's writings are considered older than the other three accounts.
Now, it doesn't stop Mark in being called to Apostleship when there was need to fill the missing position. Whether it was so, we are left to the mercy of the scanty writings of what was considered canonized. Yet, the puzzling here, only a prophet and the twelve can write scriptures.
