We investigated the effect of the cooling rate and molten metal temperature on the microstructure and mechanical properties of a sub-rapidly solidified magnesium alloy, Mg–10Al–0.2Mn–1Ca, prepared by antigravity suction casting using a water-cooled steel mold. The microstructure of the antigravity-suction-cast material without water cooling consisted of coarse grains (grain size: 780 µm), with networks of an Al–Ca compound at the grain boundaries. The higher cooling rate of the water-cooled steel mold promoted the formation of the Al–Ca compound and voids in accordance with increases in the internal and external temperature gap and differences in the solidification rate of the mold. The formation of voids and the shrinkage were suppressed, however, by adjusting the cooling rate and decreasing the molten metal temperature. The particle size of the Mg phase was refined to 135 µm and the grain-boundary compounds were finely dispersed in the Mg phase. The as-cast alloy showed an ultimate tensile strength of 166 MPa and an elongation of 8%. The microstructure and mechanical properties of the as-cast alloy were dependent on the cooling rate and molten metal temperature, but they were not dependent on the casting speed.