Accurately reconstructing the paleogeography of the Laurentide Ice Sheet (LIS) during Marine Isotope Stage 3 (MIS 3; ca. 57,000 to ca. 29,000 yr B.P.) is critical for understanding glacial growth toward the Last Glacial Maximum (LGM), refining sea-level histories, and studying the Earth system response to rapid climate change events. Here, we present a geochronological data set useful for testing hypotheses of global sea level and refining ice sheet configuration through this interval. Data (n = 735) span the entire MIS 3 interval and consist of 14C determinations (n = 651), cosmogenic exposure ages (n = 52), and optically stimulated luminescence dates (n = 32). On that basis, we hypothesize that the central region of the LIS underwent a dramatic reduction in ice from ca. 52 to 40 ka. Key to this hypothesis are geological records at sites in the Hudson Bay Lowlands (east central Canada) that suggest a marine incursion and development of terrestrial landscapes. We show that these landscapes are consistent with recently published glacial isostatic adjustment predictions that include widespread deglaciation of the eastern (Labrador) sector of the LIS with ice buildup over the western (Keewatin) sector at 42 ka. Ice growth from this minimum toward the LGM is likely to have been rapid. The agreement between this data set and modeling predictions prompts the reassessment of key Late Pleistocene records, including Heinrich events, loess deposition in the continental United States, and sedimentological records from the Gulf of Mexico.