We report a diverse assemblage of tribosphenidan mammals from several localities in the Cloverly Formation (Lower Cretaceous, Albian) of Montana and Wyoming. This unit is of historical significance for yielding well-known dinosaurs (e.g., Deinonychus antirrhopus, Tenontosaurus tilletti) and early mammals (e.g., Gobiconodon ostromi, Montanalestes keeblerorum). We provisionally identify 13 taxa (five of which are formally recognized), including Pappotherium pattersoni, a new species of the deltatheroidan Oklatheridium (O. wiblei, sp. nov.), a eutherian (Montanalestes, previously named), and two new basal tribosphenidans (Argaliatherium robustum, gen. et sp. nov., and Carinalestes murensis, gen. et sp. nov.). An unnamed taxon, represented by associated but almost edentulous dentaries, is interpreted to have had four incisors, a single-rooted canine, three premolars, and four molars, indicating that the metatherian tooth formula was established by the Albian. In addition, an indeterminate lower molar fragment preserving twinned talonid cusps and a buccal postcingulid provides the earliest evidence for Marsupialiformes. We also provide a more detailed description of the associated dentaries (holotype) of Montanalestes keeblerorum. The mammalian fauna from the Cloverly Formation shares several taxa with the roughly contemporaneous Trinity Group of Oklahoma and Texas, an observation that also applies to the dinosaur fauna, suggesting some degree of latitudinal homogeneity among described terrestrial vertebrates in this part of the North American Early Cretaceous.