The turnover of groundwater through recharge drives many processes throughout Earth's surface and subsurface. Yet groundwater turnover rates and their relationship to regional climate and geology remain largely unknown. We estimated that over 200 million km3 of groundwater has recharged since the Last Glacial Maximum (LGM), which is ten times the volume of global groundwater storage. However, flushing is very unevenly distributed throughout Earth's one million watersheds, with some aquifers turned over thousands of times to others with <1% turnover. The median global groundwater turnover of 5 ± 3 times since the LGM highlights groundwater's active role in earth system processes. Incomplete groundwater turnover since the LGM beneath a third of land areas reveals the imprint of relict climate conditions on modern-day groundwater resources. The bulk groundwater turnover calculated here enable better quantification of groundwater's impact in dynamic global water budgets and the transport of nutrients, contaminants, and geologic weathering products.