The potential of cast magnesium alloys for being used as structural materials in lightweight applications is assessed. The ability of the alloys for mechanical performance is evaluated and compared against the ability of widely used structural aircraft cast aluminum alloys. The specific quality index , devised for evaluating both cast and wrought aluminum alloys, will be exploited to evaluate the ability of a number of cast magnesium alloys for mechanical performance. The exploited quality index involves the material’s yield strength to account for strength, the strain energy density to account for both tensile ductility and toughness, and the material’s density . The effects of differences in chemical composition and heat treatment conditions on the mechanical performance of cast magnesium alloys have been assessed. The use of the quality index has been proved to appreciably facilitate the evaluation of the mechanical performance of cast magnesium alloys and also the comparison between alloys of different base materials. The results quantify the gap to be closed such as to involve cast magnesium alloys in aircraft structural applications.