The Last Interglacial (LIG), or MIS 5e (ca. 130–115 ka), is characterized by high Northern Hemisphere summer insolation and a sea-level highstand about 6–9 m higher than today. The melting of the two polar ice sheets contributed substantially to this sea-level rise but quantifying the contribution from each is difficult. The configuration of the Greenland Ice Sheet (GrIS) during the LIG and its exact contribution to sea level is not well established. Utilizing a fully coupled, fast Earth system model, we ran a 108-member ensemble of coupled climate–GrIS simulations, with a particular focus on the GrIS. We perturbed various ice sheet and geophysical model parameters to create the ensemble, with paleoproxy evidence providing constraints to validate the simulations. The constraints included elevation lowering at the NGRIP, GRIP, GISP2, and NEEM ice core sites from present day to the LIG, the simulated present-day elevation at the GRIP site, and the coverage by ice over all deep Greenland ice core locations throughout the LIG. Only two simulations passed all three primary constraint tests. Relaxing the ice-coverage constraint at Camp Century and Dye-3 results in 13 additional simulations that satisfy the constraints. These simulations result in LIG ice loss of 2.43–3.16 m sea level equivalent, accounting for over a quarter of the LIG sea-level highstand. The remainder of the highstand is unaccounted for by GrIS melt and likely comes from the Antarctic Ice Sheet. Our results demonstrate that the GrIS underwent significant but incomplete retreat during a warm period of elevated insolation, highlighting its potential vulnerability under sustained future anthropogenic greenhouse warming.