Marine based ice sheets are subject to rapid destabilisation during periods of increasing sea level and ocean temperature, especially in areas with an inland-sloping bed. Considerable portions of the East Antarctic Ice Sheet are grounded below sea level, and are thus theoretically prone to marine ice sheet instabilities. However, the response of this potentially susceptible East Antarctic ice is presently unclear, as there is limited evidence of the rate and timing of past deglacial events in such areas. We use a combination of new onshore cosmogenic dating and offshore multibeam bathymetry, alongside existing radiocarbon dating, to show that the East Antarctic Ice sheet displayed a marine ice sheet instability response following the Last Glacial Maximum, c. 20,000 years before present. We find that the ice sheet in east Prydz Bay retreated rapidly across 200 km of inland sloping bed during a period of sustained ocean warming. We also employ a numerical model that replicates the pattern and timing of retreat in Prydz Bay to demonstrate that instabilities also occurred elsewhere with similar basal topographies.