Electrolytic manganese residue (EMR) and steel slag (SS) are major industrial byproducts with distinct chemical and environmental characteristics. EMR contains high levels of leachable NH4+ and Mn2+, whereas SS is mainly composed of Ca, Si, Fe and Mg bearing phases and may also contain trace metal-bearing components. To evaluate the durability and heavy-metal leaching behavior of electrolytic manganese residue–steel slag cementitious materials (ESM), binder specimens with different EMR/SS ratios were exposed to distilled water, sodium sulfate solution, and sodium chloride solution. Their compressive strength, strength erosion coefficient, leaching characteristics, and microstructural evolution were systematically investigated. The microstructure of the specimens was also characterized by XRD, FTIR, SEM and MIP. The results indicate that an appropriate EMR/SS ratio contributed to improved strength retention under the tested exposure conditions, and specimens with EMR contents ≤ 40% showed better erosion resistance than the EMR50% group. NaCl solution had the greatest negative effect, followed by Na2SO4 solution, distilled water had the least effect, and Friedel’s salt was formed by the erosion of NaCl solution. The decomposition of the amorphous network-like gel occurred, and the dissolution of soluble substances increased the pore size of the specimens, which was the main reason for the decrease in strength. The NH4+ concentration in the ESM leachate was below the detection limit. -Mn2+ was obtained mainly in the form of H2Mn8O16·2.4H2O with a curing rate greater than 99.99%. The leaching concentration of heavy metals was in accordance with the Class IV groundwater standard as stipulated in the Chinese standard (GB/T 14,848-2017). These findings suggest that EMR–SS–GGBS binders can achieve acceptable durability and effective heavy-metal immobilization under the tested accelerated exposure conditions when the EMR content is controlled at an appropriate level.