The middle Eocene of North American was an important time in carnivoramorphan evolution, because it is when the order showed increases in diversity taxonomically and ecomorphologically. We describe new fossils of “miacoids” from the Uinta Formation of northeastern Utah, and assess their phylogenetic and paleobiological significance. A new species of Miocyon (Matthew, 1909), is described, Miocyon vallisrubrae, which, in several aspects of lower molar morphology, is intermediate between the smaller Miocyon major (Matthew, 1909) and the larger Miocyon scotti (Wortman and Matthew, 1899). In addition, new well-preserved dental and gnathic specimens of M. scotti are described, along with fragmentary cranial and postcranial remains. The taxonomy of Miocyon is reviewed in light of the new material, which highlights the divergent specializations of the genus. These include an m1 with a relatively broad talonid and an m2 that is large, broad and relatively flat with little difference in height between the trigonid and talonid. These features suggest a shift from a shearing-dominated diet to a greater reliance on grinding. An ecological analogue for the radiation of this lineage among living carnivorans may be the sympatric African species of jackals. The distribution of similar features in Miocyon as well as other “miacoids” suggests an ecomorphologically similar phylogenetic group containing Miocyon, Uintacyon Leidy, 1872, and Vassacyon Matthew, 1909. New specimens from the Uinta Formation are also described of Tapocyon Stock, 1934, Procynodictis Wortman and Matthew, 1899, a new unnamed miacid, and an unnamed small viverravid. These highlight the adaptive diversity of carnivoramorphans during the Uintan North American Land Mammal Age.