Slow climate feedbacks are currently underrepresented in model assessments of climate sensitivity and their magnitudes are still poorly constrained. We combine a recently published record of Earth’s Energy Imbalance (EEI) with existing reconstructions of temperature, atmospheric composition, and sea level to estimate both the magnitude and timescale of the ice sheet-albedo feedback since the Last Glacial Maximum. This facilitates the first opportunity to quantify this feedback over the most recent deglaciation using a purely proxy data-driven approach, without the need for simulated reconstructions. We find the ice sheet-albedo feedback to be amplifying, reaching an equilibrium magnitude of 0.55 Wm-2K-1, with a 66% confidence interval of 0.45 - 0.63 Wm-2K-1. The timescale to equilibrium is estimated as 3.6Kyrs (66% confidence: 1.9 - 5.5Kyrs). These results provide new evidence for the timescale and magnitude of the amplifying ice sheet-albedo feedback that will continue to drive anthropogenic warming for millennia to come.