We have investigated the effects of mantle phase‐transitions on the Earth's thermal evolution by numerical modelling. Our results show that the tendency of layering grows with increasing vigor of convection. A self‐consistent model which couples the convective dynamics of the mantle to the secular cooling of the core is constructed. For a temperature‐dependent effective viscosity, we found that the thermal history of the Earth is characterized by two timescales, one is associated with layered convection in the early period, which is followed by a transition period with dramatic overturns. As the Rayleigh number decreases sufficiently low, whole mantle convection prevails and a faster cooling timescale ensues.