All phase diagram and thermodynamic data of the MgO–MnO and MgO–MnO–MnO2 systems were critically evaluated and thermodynamically optimized to obtain one set of thermodynamic model parameters. The MgMn2O4–Mn3O4 spinel solutions were modeled with two sublattices Compound Energy Formalism by considering the cation distribution between tetrahedral and octahedral sites. The slag phase and monoxide solid solution were also described using the Modified Quasichemical Model and Bragg–Williams random mixing model, respectively. The optimized thermodynamic model parameters can be used with a Gibbs energy minimization routine to calculate the phase diagram and thermodynamic properties of the system at oxygen partial pressures between metal saturation and 1 atm.