A planet’s surface conditions can significantly impact its climate and habitability. In this work, we use a 3D GCM to systematically vary dayside land cover on a tidally locked M-Earth under two extreme and opposite continent configurations, in which either all of the land or all of the ocean is centred at the substellar point. We identify water vapour and sea ice as competing drivers of climate, and we identify land-dependent regimes under which one or the other dominates. We find that land fraction and distribution can cause ample differences in average surface temperature and atmospheric water vapour, with the most discrepant cases occurring at partial dayside land cover with opposite continent configuration. Since M-Earth surfaces will not be directly observable using transit spectroscopy, these differences represent a fundamental uncertainty in an M-Earth’s climate, even if the atmospheric composition is well-known. Our results are robust to variations in atmospheric CO2 concentration, stellar temperature, instellation, and surface albedo.