Dramatic changes (acceleration, thinning, and grounding-line retreat of major ice streams) in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) have been observed during the past two decades, but the millennial-scale context for these changes is not yet known. We present the first surface exposure ages recording thinning of Pine Island, Smith, and Pope Glaciers, which all drain into the Amundsen Sea. From these we infer progressive thinning of Pine Island Glacier at an average rate of 3.8 ± 0.3 cm yr−1 for at least the past 4.7 k.y., and of Smith and Pope Glaciers at 2.3 ± 0.2 cm yr−1 over the past 14.5 k.y. These rates are more than an order of magnitude lower than the ∼1.6 m yr−1 recorded by satellite altimetry for Pine Island Glacier in the period 1992–1996. Similarly low long-term rates (2.5–9 cm yr−1 since 10 ka) have been reported farther west in the Ford Ranges, Marie Byrd Land, but in that area, the same rates of thinning continue to the present day. Our data provide the first evidence that puts into context recent rates of thinning of the WAIS in the Amundsen Sea Embayment and demonstrates that these are unusually rapid. The data also provide much-needed constraints for ice sheet models, which are the primary tool for predicting the future behavior of the WAIS and its likely contribution to sea-level rise.