Mantle flow occurs through both linear and nonlinear creep mechanisms and the style of convection is strongly influenced by the dominant creep mechanism in a composite rheology. We show here by numerical modelling of convection with a composite flow law that the nonlinear, strain‐rate dependence of mantle rheology would dominate over the linear (Newtonian) portion for sufficiently vigorous convection such as might have prevailed in the early Earth. This rheological transition occurs at a Rayleigh number value which increases dramatically with higher values of the activation energy. For activation energy values for dislocation and diffusion creep of about 100 and 70 kJ/mole respectively we find a transition between Rayleigh numbers 106 and 107. Corresponding values of the transition‐stress agree with estimates for the lower mantle from empirical data for perovskite analogue. These results suggest that early in the Earth's thermal history a transition would have taken place from a primarily non‐Newtonian flow to a Newtonian, temperature‐dependent style of mantle convection. With a high activation energy, greater than 125 kJ/mole, the mantle would have been in a Newtonian regime throughout most of its thermal history.