The last deglaciation in the Northern Hemisphere was interrupted by two major stadials, the so-called “Mystery Interval” (17.5–14.5 ka) and the Younger Dryas (12.9–11.7 ka). During these events, the North Atlantic region was marked by cold surface conditions, yet simultaneous glacier and snowline retreat. Rerouting of Laurentide Ice Sheet meltwater from the Gulf of Mexico to an eastern or northern spillway may have reduced meridional overturning circulation at the onset of the Younger Dryas. However, this hypothesis has not been tested for the Mystery Interval. Paired Mg/Ca and δ18O measurements on foraminifera from laminated Orca Basin sediments in the Gulf of Mexico, constrained by 35 14C dates, document the timing of meltwater input with subcentennial resolution. Isolating the δ18O of seawater (termed δ18OGOM) reveals three major melting episodes from ca. 17.5 ka until 12.9 ka, followed by a rapid cessation, consistent with meltwater rerouting at the onset of the Younger Dryas. Conversely, inferred meltwater flow to the Gulf of Mexico during the Mystery Interval does not support a simple routing event, but is consistent with glacier and snowline retreat. We suggest that summer melting of Northern Hemisphere ice sheets during this stadial may have been an important mechanism for enhanced winter sea-ice formation, hypercold winter conditions, and enhanced seasonality in the North Atlantic region.