A scanning electron microscopy (SEM)-based electron backscatter diffraction (EBSD) analysis showed that the discontinuous precipitation (DP) reaction rate was dependent on the geometry of the grain boundary in Mg–9Al–1Zn (wt.%) alloys. DP converted a supersaturated solid solution, 𝛿₀ (magnesium (Mg)-rich solid solution), into a two-phase 𝛿 + 𝛾 aggregate, with 𝛿 being a precipitated Mg₁₇Al₁₂ (intermetallic phase) behind a migrating reaction front. The near-special grain boundary was rather inactive, whereas most of the random high-angle boundaries promoted the reaction. Prior deformation (hot rolling to achieve up to 80% thickness reduction) had no effect on the frequency of special-grain boundaries.