Cast Al–Mg alloys with high Mg content have attracted considerable interest as a result of exceptionally low mass density and enhanced Mg solid-solution strengthening effect. However, poor ductility in this class of cast Al–Mg alloys originating from the increased amount of brittle Al3Mg2 phase presents a challenge. This work aims to improve ductility of a cast Al-10 wt%Mg alloy by prolonging time and/or increasing temperature during solid-solution treatment, hence minimizing the amount of the Al3Mg2 phase. The amounts of Al3Mg2 decrease and the concentrations of solute Mg increase with prolonging time and/or increasing temperature, improving both ductility and strength. Solid-solution treatment at 425 °C for 9 h or at 438 °C for 3 h induces complete dissolution of the Al3Mg2 phase, achieving an average uniform elongation of ~27% accompanied by an average 0.2% offset yield strength of ~186 MPa. The mechanisms improving both ductility and strength are discussed.