We reconstruct terrestrial and freshwater environments of the last two Quaternary interglacials in coastal east Antarctica by examining multi-proxy evidence in a lake sediment core. The record, from Progress Lake in the Larsemann Hills consists of two discrete sediment units. The lower unit contains the first known evidence of Antarctic coastal environments during a previous interglacial, likely to be MIS5e. Biogeochemical, biological and sedimentological climate proxies revealed a productive biological community and an active hydrological regime, consistent with the warmer conditions detected in continental ice cores at this time. The diatom assemblage was similar to that currently found in the sub- and maritime-Antarctic biome and included some sub-Antarctic endemic taxa that have not previously been reported from east Antarctica. This suggests southward expansion of the sub-Antarctic diatom biome during MIS5e. Mosses were also present in the lake or the catchment but are rare in the region today. Two discrete periods of enhanced productivity show that the MIS5e environment was not stable. MIS5e ended abruptly with a rapid transition to glacial conditions, the lake was covered by a layer of firnified snow and ice, and phototrophic biological activity ceased for a period of c. 90,000 years. The upper unit was deposited in MIS1 after 3470–3687 cal yr BP after phototrophic biological production resumed. Lower species diversity, pigment production, an Antarctic continental diatom assemblage and an absence of moss layers suggest cooler conditions during MIS1.