In this study, an overcasting process followed by a low-temperature (200°C) annealing schedule has been developed to bond magnesium to aluminum alloys. ProCAST software was used to optimize the process parameters during the overcasting process which lead to Mg/Al bimetallic structures to be successfully produced without formation of Mg-Al intermetallic phases. Detailed microstructure evolution during annealing, including the formation and growth of Al-Mg interdiffusion layer and intermetallic phases (Al₁₂Mg₁₇ and Al₃Mg₂), was experimentally observed for the first time with direct evidence, and predicted using Calculation of Phase Diagrams (CALPHAD) modeling. Maximum interfacial strength was achieved when the interdiffusion layer formed at the Mg/Al interface reached a maximum thickness the without formation of brittle intermetallic compounds. The precise diffusion modeling of the Mg/Al interface provides an efficient way to optimize and control the interfacial microstructure of Mg/Al bimetallic structures for improved interfacial bonding.