"Mark 16:9-20 is original to Mark's Gospel."
Almost no textual scholar today regards the long ending as original.
Evidence card
The rating is tied to the stated evidence, the stated caveat, and the named source trail below.
What's actually true
The long ending is found in Codex Alexandrinus (~AD 425), Bezae, the Vulgate, and the Byzantine majority text.
What gets left out
It is missing from the two oldest complete codices (Sinaiticus, Vaticanus), from the Old Syriac, from many early Armenian and Georgian manuscripts, and from a number of Church Fathers (Eusebius, Jerome) who explicitly note its absence. The Greek style differs from the rest of Mark. Modern translations (NIV, ESV, NRSV, NET, CSB) all bracket or footnote it. 'Probably original' is no longer a defensible position.
Source trail
Named scholarship, catalogues, or projects used for this claim rating.