The characteristics of Greenland ice core stadial 20 (GS-20), the first stadial of Marine Isotope Stage 4 at the onset of the most recent glacial period, have been linked to atmospheric aerosol forcing associated with the massive 74 ka Toba (Indonesia) supereruption. More precise 40Ar/39Ar dates are available for the timing of the Toba supereruption, but while greatly improved, ice core records are not tied to absolute chronologies over this specific time interval to test for synchronicity. Here we show a rapid transition to drought conditions in the southwestern United States from a rise in a moisture-sensitive δ13C time series of stalagmite KR1 from southwestern New Mexico (USA) at precisely 74.19 ± 0.22 ka. The rapid rise in δ13C in stalagmite KR1 is synchronous with the Toba supereruption 40Ar/39Ar dates within 2σ absolute age error, and shows a distinct peak that started and ended abruptly, and lasted ∼1560 yr. The δ18O time series follows Northern Hemisphere summer insolation, and the absence of a prominent δ18O anomaly at 74 ka suggests that the atmospheric response in the southwestern United States was dominated by moisture and atmospheric CO2, not temperature or moisture source variations. The coincidence of the initiation of southwest aridity at the time of the Toba supereruption is consistent with an interpretation that the eruption caused far-reaching climate change; however, climatic anomalies such as this Greenland stadial would thus likely require long process-time feedback systems related to glacial thresholds to sustain the less than decadal-long volcanic-induced atmospheric interferences. The debate regarding the role of the Toba supereruption as a possible trigger of GS-20 requires robust chronologies to resolve and advance our understanding of the potential climatic triggers and feedbacks related to this event. Our chronology offers an independent tie to the Greenland ice core record and supports the possibility of a Toba supereruption trigger of GS-20.