The transition from the glacial period to the Holocene was characterized by a dramatic reorganization of Earth’s climate system linked to abrupt changes in atmospheric and oceanic circulation. In particular, considerable effort has been placed on constraining the magnitude, timing, and spatial variability of climatic changes during the Younger Dryas stadial (YD; 12.9–11.7 ka) within the North Atlantic region. Whereas the YD is clearly expressed in some climate archives, the record of mountain glacier change through the YD remains enigmatic and has elicited debate concerning the overall magnitude and seasonality associated with YD temperature change. Here, we report 19 new 10Be ages from a location in the Pacific sector—the Ahklun Mountains, southern Alaska—that constrain the age of a late-glacial terminal moraine to 12.52 ± 0.24 ka, in the middle of the YD stadial. Our new 10Be ages, combined with additional Northern Hemisphere records of glacier change, reveal that glacier culminations occurred in the early and/or middle YD, followed by glacier recession through the remainder of the YD. Widespread early-to-middle Younger Dryas glacier culminations imply modest summer cooling (i.e., seasonality) that briefly punctuated an overall warming trend through the YD stadial. This pattern of glacier culminations occurring in the early-to-middle YD followed by retreat through the remainder of the YD largely mimics the pattern of YD temperature change displayed in Greenland ice cores.