The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated δ13C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive δ13C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian δ13C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric pCO2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level—which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of pCO2 by suppressing shelf-carbonate production—signals the transition to a Late Ordovician icehouse climate ∼10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.