Tatian’s Diatessaron, a harmonized gospel composed c.170 ce, is often cited as one of the earliest explicit references to the Longer Ending (LE) of Mark (16:9–20). Yet no comprehensive study of the LE’s presence in the Diatessaron has been carried out, and there is much confusion over its use as evidence. The current study compares the resurrection narrative in the eleventh-century Arabic harmony with that of the sixth-century Latin Codex Fuldensis, the two earliest and most reliable representatives for reconstructing the Diatessaron’s sequence. If they incorporate the LE in exactly the same way, we may safely conclude that Tatian’s copy of Mark contained the LE. Using neighboring harmonies as controls, I arrive at two parallel conclusions: (1) Tatian almost certainly incorporated significant portions of Mark 16:9–20 into his Diatessaron; (2) it is not entirely clear which portions of Mark 16:1–8 were present in the Diatessaron. Ultimately, the study demonstrates that Tatian’s Diatessaron is likely the earliest uncontested external evidence for the LE.