<p>Improving the mechanical properties of magnesium alloys for use in lightweighting applications is a critical strategy for global emission reduction and ecological sustainability. In this study, the microstructure, mechanical properties and heat treatment response of a permanent mold gravity cast Mg-5Zn-5Al-0.25Si were investigated. Microstructural examination identified three distinct secondary phases, ϕ-Mg₂₁(Zn, Al)₁₇, γ-Mg₁₇Al₁₂ and Mg₂Si. Moreover, preferential precipitation of ϕ-Mg₂₁(Zn, Al)₁₇ over brittle γ-Mg₁₇Al₁₂ intermetallics was observed. Tensile testing results indicated that the Mg-5Zn-5Al-0.25Si alloy had higher levels of strength and ductility relative to other commonly used magnesium alloys. Solution heat treatment was effective in dissolving both the ϕ-Mg₂₁(Zn, Al)₁₇ and γ-Mg₁₇Al₁₂ phases, but not the Mg₂Si phase. The alloy also displayed a noticeable age-hardening response when exposed to artificial aging conditions, which led to a 32% increase in hardness at the peak aged condition. Thus, the Mg alloy developed in this study will promote increased use in industry.</p>