The continental response to global climate forcing remains difficult to predict due to the intricate feedbacks among climate, vegetation and other land surface changes. This is exemplified by the Eocene-Oligocene Transition (EOT; 34 Ma), where a 2-3°C cooling event is observed in marine records around the world, but continental records from the North American Great Plains suggest more extreme cooling of 7-8°C instead. Here we present a new Oligocene record from the adjacent high-elevation North American Cordillera (Cook Ranch section; SW Montana) constrained by four radiometrically dated tuffs. Dual clumped isotopes (Δ47 and Δ48) of this record suggest no major changes in temperature across the EOT, but instead show extreme cooling of 10±1°C in the early Oligocene (32-30 Ma). Based on paleobotanical and climate model data, we interpret this as summer cooling coeval with a decrease in atmospheric CO2 identified in recent proxy compilations. This long-term Oligocene cooling may explain the lack in mammal response observed in North American fossils compared with other continents.