The St. Mary River Formation (Fm) crops out in restricted parts of southwestern Alberta and northwestern Montana. Mammals are poorly known from the formation, but material collected from Alberta has played a role in recognizing an ‘Edmontonian’ Land Mammal Age between the better sampled Judithian (∼79–74 Ma) and Lancian (∼67–65 Ma) land mammal ages. New and well-preserved multituberculate and metatherian material collected from a single locality in the lower third of the St. Mary River Fm of Montana adds complexity to the interpretation of a discrete ‘Edmontonian’ Land Mammal Age. We report here three taxa of multituberculates (Paracimexomys propriscus, sp. nov., Nidimys occultus, gen. et sp. nov., and a primitive ptilodontoid of comparable size to the smallest species of Mesodma Jepsen, 1940, and Cimexomys Sloan and Van Valen, 1965) and two metatherian taxa (Leptalestes toevsi, sp. nov., and Turgidodon russelli). Paracimexomys propriscus and Turgidodon russelli are probably conspecific with Judithian forms that did not survive to the Lancian. Leptalestes toevsi and Nidimys occultus are taxa unique to the ‘Edmontonian’ but may represent lineages that separated from their closest relatives prior to the Judithian. Lack of Lancian-aspect mammals contrasts sharply with other ‘Edmontonian’ faunas, including an older fauna recovered from the Williams Fork Fm of Colorado, suggesting that the replacement of Judithian mammals by Lancian forms was a complex transition. We tentatively suggest that the Lancian fauna may have expanded into the northern part of the Western Interior in response to the appearance of new terrestrial habitats as sea level fell.