The effects of precipitate distribution in commercial Mg alloys on the discharge behavior of a Mg–air battery were investigated. Rolled Mg–9 mass%Al–1 mass%Zn (AZ91) alloy sheets were selected as the anode. The discharge behaviors were evaluated by constant current discharge tests, and the corrosion behaviors were estimated by salt immersion tests and potentiodynamic polarization measurements. In the discharge tests, the peak-aged specimens exhibited much shorter discharge time than the solution-treated specimen. In the corrosion tests, the aged specimens exhibited quite a low corrosion rate as compared with the solution-treated specimen. From the potentiodynamic polarization measurements, the aged specimens showed a much lower corrosion current density and higher corrosion potential than the solution-treated specimen. It is suggested that a stable corrosion barrier by densely distributed β-phase precipitates contributed to both the reduction of discharge time and the suppression of corrosion rate for the aged specimens. Microstructural evaluations of the aged specimens revealed that fine β-phase precipitates were densely distributed throughout the specimens and that there was little Al-rich α phase, which would accelerate galvanic corrosion between the α matrix and the β phase. It was confirmed that the finely and homogenously distributed β phase in the aged AZ91 specimens acted as a barrier to the dissolution of the α-Mg matrix because a large Al-rich α phase was not distributed in the AZ91 specimens in this study.