Abstract: Mg–Zn–Mn alloys have the advantages of low cost, excellent mechanical properties, and high corrosion resistance. To clarify the phase equilibria of Mg–Zn–Mn alloy in the Mg-rich corners, the present work experimentally investigated the phase equilibria in the Mg-rich corner at 300–400°C with equilibrated alloy method using electron probe micro analyzer (EPMA), X-ray diffractometer (XRD), transmission electron microscopy (TEM), and differential scanning calorimeter (DSC). Mn atoms were found to dissolve into MgZn₂ to form a ternary solid-solution type compound, in which Mn content can be up to 15.1at% at 400°C. Three-phase equilibrium of α-Mg + MgZn₂ + α-Mn and liquid + α-Mg + MgZn₂ were confirmed at 400°C. Subsequently, thermodynamic modeling of the Mg–Zn–Mn system was carried out using the CALPHAD method based on the experimental data of this work and literature data. The calculated invariant reaction Liquid + α-Mn → α-Mg + MgZn₂ at 430°C shows good agreement with the DSC results. In addition, the results of solidification path calculations explain the microstructure in the as-cast and annealed alloys well. The agreement between the calculated results and experimental data proves the self-consistency of the thermodynamic database, which can provide guidance for the compositional design of Mg–Zn–Mn alloys.