The behaviour of ice caps and glaciers on sub-Antarctic islands during previous periods of warming provide key empirical evidence for understanding the behaviour of marine ice sheets in the future. However, the extent of ice on sub-Antarctic islands during the last 100 kyr is poorly constrained. Here, we investigate the past glacial extents on South Georgia, where previous Last Glacial Maximum (LGM) reconstructions vary between small fjord-terminating glaciers and a large marine-based ice sheet. To help resolve this uncertainty, we apply Schmidt hammer relative-age dating to measure rock hardness and, thus, exposure age of a range of glacial deposits. Applying a hardness–age calibration curve constructed from well-dated Holocene, late-glacial deposits and terminal LGM deposits, we determine that deglaciation of the approximately 600 m-high peaks on the outer Lewin Peninsula occurred during the latter half of the last glacial stage, and probably the end of the LGM. We infer that South Georgia was covered by a marine-based ice cap during the latter part of the last glacial stage.