Glacial sediments from the Prince Charles Mountains, East Antarctica, record late Pleistocene ice thickness variability in the Lambert Glacier–Amery Ice Shelf system, one of the world's largest ice drainages. A former glacial limit, demarcated by minimally weathered deposits, follows a concave longitudinal profile, indicating a zone of strong ice streaming through the northernmost 500 km of the Lambert Graben. In situ 10Be and 26Al exposure ages from these relatively unweathered deposits indicate that the most recent phase of ice lowering occurred between ca. 18 and 8 ka, preceding by as many as 6 k.y. the deglaciation of adjacent coastal regions. Earlier onset of deglaciation in an area of strong ice streaming suggests a heightened sensitivity of the East Antarctic Ice Sheet to climate and sea-level changes following the Last Glacial Maximum than previously recognized.