Almost all contemporary New Testament textual critics have concluded that neither the longer or shorter endings were originally part of Mark’s Gospel, though the evidence of the early church fathers above shows that the longer ending had become accepted tradition. The United Bible Societies' 4th edition of the Greek New Testament (1993) rates the omission of verses 9-20 from the original Markan manuscript as "certain." For this reason, many modern Bibles decline to print the longer ending of Mark together with the rest of the gospel, but, because of its historical importance and prominence, it is often included as a footnote or an appendix alongside the shorter ending. Nevertheless, a handful of scholars, particularly those in traditionalist or fundamentalist traditions, argue that the evidence is insufficient to justify its exclusion or that the evidence in fact supports its inclusion. However, in biblical scholarship, changes and advances due to creative detective work and new discoveries have a long past history of proceeding with caution very slowly, so the almost unanimous conclusion with regards to the inauthenticity of the ending(s) of Mark should be seriously considered.
Against the above: In 177 AD Irenaeus wrote Against Heresies. In it he cites from Mark 16:19, establishing that the longer reading was in existence at this time and was considered canonical, at least by Irenaeus. Quoting , "The difference here is extremely important. If we conclude that this passage is not authentic, then we must question what happened to the original ending of Mark. It is not logical that the Gospel would end at this place so abruptly. Nor is it likely, as some scholars have suggested, that the Gospel was never finished, calling biblical inspiration into question. The conclusion held by most textual scholars, whether liberal or conservative, that the original ending has been lost over the passage of time certainly denies the doctrine of biblical preservation. If we allow that a passage of inspired Scripture has been lost from this section of the Bible, what stops us from making the same application to other passages? It is certainly within the realm of scholastic studies to note any and all textual differences. But once we open the possibility that this or that passage has been lost, we are now trusting in the understanding of men over the biblical promises of God. Certainly it is better to embrace the textual evidence and hold to the promise of preservation."
See also: About the Shorter and Longer Endings of Mark 16 (Mark 16 footnotes, New American Bible, United States Conference of Catholic Bishops) Excerpt: "vocabulary and style indicate that it was written by someone other than Mark"