This study reports for the first time the synthesis of syntactic composite foams based on magnesium matrix and hollow fly ash cenosphere particles using the powder metallurgy technique involving hybrid microwave sintering. The effects of varying amounts of hollow cenosphere particle addition on the density, microstructure, thermomechanical and mechanical properties of pure magnesium were investigated. Density measurements indicated up to 18% reduction in density with the highest quantity of cenospheres (15%) addition to the magnesium. Microstructural examination demonstrated evenly distributed, intact and a few broken cenosphere particles with good interfacial integrity. Phase analysis showed finely distributed intermetallic phases such as MgO and Mg2Si due to the reaction between Mg matrix and the hollow cenosphere reinforcement. Mechanical property measurements showed retention/slight improvement in specific mechanical properties of magnesium due to hollow fly ash cenosphere particles. However, the tensile ductility was marginally affected.