Abstract Mg and Al were joined through diffusion bonding using a silver interlayer film prepared by magnetron sputtering. The interfacial microstructure and mechanical performance of Mg/Al joints were greatly improved in the presence of the silver interlayer, which prevented the formation of brittle Mg–Al compounds. The typical microstructure at the joint interface is a Mg/Mg3Ag/MgAg/Al multilayer structure. The bonding temperature and holding time were optimized to avoid the formation of Mg–Al compounds. At low bonding temperatures (380–420°C) and short holding times (15–45min), the Mg–Al intermetallic compounds were eliminated owing to the presence of Mg–Ag compounds. The thicknesses of the Mg3Ag and MgAg layers increased with increasing temperature and longer holding times. The addition of the silver interlayer greatly improved the shear strength of the joints. The maximum microhardness of the interface compounds decreased from 368HV to 125HV owing to the absence of Mg–Al compounds. Fractures in the joints occurred between Mg3Ag/MgAg, MgAg/Al boundaries and within the inner Mg–Al layers. The fracture features differed depending on whether Mg–Al compounds were present.