Acts 12:25 And Barnabas and Saul returned from Jerusalem when they had fulfilled their ministry, and they also took with them John whose surname was Mark.
Acts 13:13 Now when Paul and his party set sail from Paphos, they came to Perga in Pamphylia; and John, departing from them, returned to Jerusalem.

How easy it is to blame John Mark for not fulfilling his obligation.
It does appear he left before things got started.
The reason is unknown but sufficient enough for Paul not to want him to travel with them later, although Barnabas did.
Perhaps foreign missionary work was not John Mark’s calling.
You cannot blame him for not doing something he is not spiritually equipped to do.
Since he returned to Jerusalem and is the author of the Gospel of Mark, it makes me believe he learned about those things from the apostles living in Jerusalem.
It all boils down to this – don’t blame someone for not doing the job you think they should do. God just may be preparing them for something better.