The influences of silicon addition to commercially pure magnesium (CP Mg) and cooling rate during solidification on the as-cast microstructure and shear mechanical properties of Mg–Si alloys were systematically investigated. For this purpose, the Mg–0.6Si, Mg–1.34Si, and Mg–3Si (wt%) alloys were considered as hypoeutectic, eutectic, and hypereutectic alloys, respectively. By decreasing the geometrical modulus of the solidifying section (increasing cooling rate), remarkable grain refinement, refining the dendrite arm spacing (DAS), and modification of Mg₂Si particles were achieved. Moreover, the grain size was refined via Si addition in the hypoeutectic range, while coarsening of grain size at high Si concentrations was observed. The results of shear punch testing and hardness measurements demonstrated that the ultimate shear strength (USS) and hardness increased by increasing the cooling rate during solidification. Moreover, Si addition generally improved hardness, while the highest USS level was achieved for the eutectic alloy due to the fine grain size and strengthening effect of the eutectic constituent. However, regarding the hypereutectic Mg–3Si alloy that exhibited high hardness, the shear properties were inferior due to the detrimental effect of the primary Mg₂Si particles. Finally, the results were discussed with consideration of the relationship between strength and hardness, for which the critical effect of Si was clarified.