Felids are keystone predators in modern ecosystems and likely played a similar role in shaping ecosystem through the Cenozoic. Unfortunately, understanding the paleoecological impact of felids has been hampered by taxonomic confusion and uncertainty in big cat diversity through time. This stems, in part, from the importance of dentition in differentiating feliform taxa combined with the abundance of postcrania in the fossil record. This dilemma is best illustrated by the humeri of a very large felid uncovered from a number of Hemphillian-aged localities in North America that, in the absence of craniodental remains, have not previously been definitively identified. We present the results of an analysis of the potential of isolated distal humeri to identify feliform taxa. We compared the form and size of distal humeri across feliforms using geometric morphometric analysis and used linear measurements to reconstruct body mass and estimate prey size. Our results suggest that distal humeri are useful tools for differentiating feliform families, genera, and species. Further, along with a specimen from Idaho with associated dentition, they suggest that the large Hemphillian-aged felid is a new species of Machairodus. Future phylogenetic analyses will be necessary to reconstruct the evolutionary history of North American Machairodontinae, but this new taxon provides evidence for a diverse predator fauna during the late Miocene of North America that included some of the largest felids in Earth history.