To discuss mantle evolution in Mercury, I present two‐dimensional numerical models of magmatism in a convecting mantle. Thermal, compositional, and magmatic buoyancy drives convection of temperature‐dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature. Magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix. Widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4.5 Gyr calculated history. The stratified structure persists for 4.5 Gyr, when the reference mantle viscosity at 1573 K is higher than around 1020 Pa s. The planet thermally contracts by an amount comparable to the one suggested for Mercury over the past 4 Gyr. Mantle upwelling, however, generates magma only for the first 0.1–0.3 Gyr. At lower mantle viscosity, in contrast, a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0.3–0.8 Gyr. Convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size. These models, however, predict larger contraction of the planet. Coupling between magmatism and mantle convection plays key roles in mantle evolution, and the difficulty in numerically reproducing the history of magmatism of Mercury without causing too large radial contraction of the planet warrants further exploration of this coupling. Key Points Coupling between magmatism and mantle upwelling in Mercury is simulated The mantle structurally evolves owing to magmatism and convective stirring The coupling causes episodic magmatism when mantle viscosity is sufficiently low