Lakes beneath the Antarctic Ice Sheet are of fundamental scientific interest for their ability to contain unique records of ice sheet history and microbial life in their sediments. However, no records of subglacial lake sedimentation have yet been acquired from beneath the interior of the ice sheet, and understanding of sediment pathways, processes and structure in subglacial lake environments remains uncertain. Here we present an analysis of seismic data from Subglacial Lake Ellsworth, showing that the lake bed comprises very fine-grained sediments deposited in a low energy environment, with low water- and sediment-fluxes. Minimum sediment thickness is 6 m, the result of prolonged low sedimentation rates. Based on the few available analogues, we speculate this sediment age range is a minimum of 150 ka, and possibly >1 Ma. Sediment mass movements have occurred, but they are rare and have been buried by subsequent sedimentation. We present a new conceptual model of subglacial lake sedimentation, allowing a framework for evaluating processes in subglacial lake environments, and for determining future lake access locations and interpreting subglacial lake samples.