Magnesium (Mg) is the lightest structural metal. However, the poor formability of Mg alloys to great extent limits their applications in making structural parts. Formability is strongly correlated to both high tensile elongation and large work hardening capacity. Here, we report a new Mg−Al−Ca alloy in which a majority of deformable Al2Ca precipitates form while the formation of Laves phases of Mg17Al12 and Mg2Ca seems suppressed. Al2Ca precipitates impede dislocation motion, leading to large work hardening. Then, Al2Ca precipitates deform with dislocations and stacking faults under the enhanced flow stress, which relieve local stress concentration and improve tensile elongation. In addition, solutes Al and Ca suppress twin nucleation while promoting 〈c + a〉 dislocations in Mg. This new Mg−Al−Ca alloy demonstrates one of the highest combinations of tensile elongation and work hardening capacity among existing Mg alloys. Display omitted