I don't believe Mark was a direct eyewitness. Wasn't he brought into the Church by Peter?
Yet his Gospel was said to have been written between 55 and 70 A.D.
None of the gospels mentions the destruction of the temple and the Romans destroying Jerusalem in A.D. 70. That was a pretty big deal. This suggests that the Gospels were all finished before 70 A.D.
Mark was an eye witness. The Last Supper is believed to have been held at his home. Also notice Mark 14:51-52.
Many Bible secholors believe that Judas first brought the soldiers who arrested Jesus in the garden to Mark's house. That would be very logical since that was the last place where Judas had known Jesus was. That being the case, it is further believed that young Mark, probably a teenager at the time, ran in his bedclothes to warn Jesus. Many believe that he is the young man who ran from the Garden naked. Only Mark records this. Mark was also probably one of the ones who witnessed the resurrected Christ.
Wasn't he brought into the Church by Peter?
No. Read Acts 12:12 When Peter was released from jail he went to Mark' house. In Acts 12:25 we find Mark and Barnabas as companions of Paul when he returned to Antioch from Jerusalem.
Read Acts 15:37-40. Here we see a dispute between Paul and Barnabas over Mark. Mark had started out with them on the first missionary journey but had not finished the journey. Now Barnabas wants to take him again on the second missionary journey but Paul says no. This resulted in Paul and Barnabus going on separate journeys, Mark going with Barnabas.
Read Col. 4:10. Here we find that Mark was with Paul when Paul wrote Colossians. They had evidently reconciled by that time.
Read II Tim. 4:11. This was written just before Paul's death. Notice that he ask Timothy to bring Mark with him when he comes to Paul in Rome because by this time Mark had grown into a strong advocate for the church and Paul considered him useful for the ministry.
The Scriptures associate Mark with Paul rather than Peter. Mark was without a doubt an eyewitness to many of the things he would later write about in his Gospel and a companion of Paul.
I hope you find this useful.