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Abstract The Laramide province is characterized by foreland basin partitioning through the growth of basement arches. Although variable along the western U.S. margin, the general consensus is initiation of this structural style by the early Campanian (~80 Ma). This has been linked to flat‐slab subduction beneath western North America, but the extent and cause for a flat slab remain debated, invoking the need for better constraints on the regional variations in timing of Laramide deformation. We present new conglomerate clast composition, sandstone petrographic, and detrital zircon U‐Pb geochronologic data from the Upper Cretaceous Beaverhead Group in southwestern Montana that suggest a pre‐Campanian history of basement‐involved deformation. During the early stages of deposition (~88–83 Ma), two separate depositional systems derived sediment from the Lemhi subbasin and distal thrust sheets to the west as well as Paleozoic strata eroding off the exhuming Blacktail‐Snowcrest arch to the east. Our data provide the first conclusive evidence for the longitudinal transport of gravel via Cordilleran paleorivers connecting sediment sources in east central Idaho to depocenters in southwestern Montana and northwestern Wyoming. Furthermore, erosion of Paleozoic strata by this time requires that the Blacktail‐Snowcrest arch was exhuming prior to ~88 Ma in order to remove the Mesozoic overburden. Later (~73–66 Ma) sediment flux was entirely from the foreland‐propagating fold‐thrust belt to the west. These results suggest that Laramide‐style deformation in southwestern Montana preceded initiation elsewhere along the margin, requiring revision of existing models for Laramide tectonism.