The Lewis thrust, which is >225 km long and has a maximum displacement of >80 km, is a major Foreland belt structural element in the southeastern Canadian Cordillera. We use low-temperature thermochronometry in the preserved Lewis thrust sheet stratigraphic succession to constrain variations in both paleogeothermal gradient and Lewis thrust sheet thickness immediately prior to motion on the Lewis thrust fault. Fission-track and vitrinite reflectance data combined with stratigraphic data suggest that maximum Phanerozoic burial and heating occurred in the Lewis thrust sheet during a short interval (<15 m.y.) in late Campanian time immediately prior to thrusting (ca. 75 Ma). The data suggest that the late predeformational Lewis thrust sheet paleogeothermal gradient was between ∼18 and 22.5 °C/km, which is higher than that inferred for subsequent syn- and postdeformational intervals by other studies. The inferred paleotemperatures and geothermal gradients indicate that the preserved Lewis thrust sheet stratigraphic succession was overlain by ∼4–5.5 km of additional Late Cretaceous strata that were subsequently removed by erosional denudation. We estimate that the Lewis thrust sheet was ∼12–13.5 km thick when thrusting commenced. Deposition of the Late Cretaceous succession was terminated by the onset of displacement on the Lewis thrust (ca. 75 ± 5 Ma) and was followed by intervals of erosional denudation that are constrained stratigraphically by both early Oligocene and current erosion surfaces on the Lewis thrust sheet.