The effects of solution heat treatment and aging on the microstructural evolution and mechanical behavior of a squeeze-cast (SC) Mg-10Gd-3Y-0.5Zr (GW103K) alloy, processed using various applied pressures (e.g., 0.1, 40, 80 and 160 MPa) were systematically investigated. Our results show that, after solution heat treatment, secondary phases and pressure-induced dislocations are dissolved in the matrix of the squeeze-cast alloys. Moreover, subsequent aging heat treatment leads to an increased age-hardening response relative to that in squeeze-cast GW103K and this trend increases with increasing applied pressure. The room temperature tensile test results show that the yield strength (YS) for the squeeze-cast alloy in the as-cast, the as-T4 heat-treated and the as-T6 heat-treated states increases with increasing applied pressure, from 0.1 to 80 MPa, and remains relatively constant when the applied pressure is increased to 160 MPa, whereas the ultimate tensile strength (UTS) and elongation-to-failure (E (f)) increases continuously with increasing applied pressure. The measured increases in YS and UTS (or E (f)), are discussed in terms of the mechanisms that govern the evolution of microstructure in squeeze-cast GW103K, paying particular attention to gain size and porosity.