While the Gospel of Mark is the first of the completed gospel accounts that has come down to us, it was not the first gospel account that was written. It is also not the first of those gospel accounts that we can recover.
Embedded within the Gospel of Matthew we can discover a work we can call Matthew’s Message. Embedded within the Gospel of Luke we can discover a work we can call Luke’s History. Embedded within Luke’s History we can discover Luke’s primary source, which we can call his #1 Source. We know that Matthew’s Message and Luke’s #1 Source predated the Gospel of Mark. While there was nothing stopping Mark’s Gospel and Luke’s History being produced concurrently, Luke’s History was probably produced just before the Gospel of Mark.
After the Gospel of Mark was produced, with its striking introductory words – “The beginning of the gospel of Jesus Christ, the Son of God” – it was not so long before it was realized that Matthew’s Message and the Gospel of Mark would work as a harmoniously single text. Hence the Gospel of Matthew was produced, probably receiving the endorsement of the great publicist, Ignatius, the early second century bishop of Antioch in Syria. When published, it borrowed its title of “Gospel” from the introductory words of the Gospel of Mark.
After the appearance of the Gospel of Matthew, it was natural to also take Luke’s masterpiece and supplement it with text taken from the Gospel of Mark, but leaving Luke’s text intact where these works overlapped. When the completed work was published, it borrowed the title “Gospel” from the Gospel of Mark and the Gospel of Matthew.
Here is an easily understood timeline chart of the likely origins of the Gospel of Matthew, the Gospel of Mark and the Gospel of Luke.