The locations, shapes and masses of evolving ice sheets during interglacial to glacial transitions remain elusive, but need to be clarified for identification of the climatic feedbacks that amplified low-amplitude insolation changes to become the major global climatic shift of the last glacial maximum. The best way to explore probable spatial patterns of ice sheet inception and build-up is through ice sheet modelling, and validating model output against geomorphological and geological data. We have modelled the 120–55 kyr BP evolution of the Laurentide Ice Sheet, focusing on the locations, outline, flow patterns, and basal thermal regimes of its precursor ice centres. For this, we employed a time-dependent mass-balance-driven finite element model, forced with GRIP ice core oxygen isotope values calibrated to temperature. The model achieves a good fit to relict ice flow systems in Keewatin, the Interior Plains, and the Hudson Bay lowland. Thus, our model indicates that the first coherent ice centre in North America was situated over Baffin Island and the Melville and Boothia Peninsulas, and expanded onto mainland Canada at 90–75 kyr BP. A previously unknown 200-km-long moraine zone near the Thlewiaza River, Keewatin, may demarcate the marginal position of this early Central Arctic Ice Sheet. Cordilleran and Keewatin ice merged, and the Hudson Bay lowland was inundated from the east (from Quebec), by 65 kyr BP.