The net influx of water into the deep mantle by plate tectonics has been poorly constrained because it is difficult to quantify how efficiently subducting slabs are devolatilized on a global scale. The significance of deep water cycle in the Earth history is similarly ambiguous because it depends critically on when plate tectonics started and how it evolved through time. Here I show that, using the new scaling of plate-tectonic convection based on fully dynamic calculations, the thermal evolution of Earth consistent with geochemical, petrological, and geological data requires continuous mantle hydration since the early Earth, with the net water influx of ∼2–3 × 1014 g yr−1. A drier mantle in the Hadean and Archean is suggested to help the initiation of plate tectonics by reducing the viscosity contrast between lithosphere and asthenosphere. As an increase in the vigor of plate tectonics with time would encourage global marine inundation, the slow intake of surface water by the convecting mantle is essential to maintain the continental freeboard.