Rechargeable magnesium batteries (RMBs) are an emerging class of high-energy storage system. The appeal of RMBs lies in the doubly-charged Mg2+ cation offering a higher volumetric capacity (3382 mAh/cm³) compared with lithium-ion batteries (2262 mAh/cm³), combined with high abundance, low cost, low toxicity, reduced risk of dendritic growth, ease of handling, and negative electrode potential of -2.35 V vs SHE. However, the instability of Mg metal negative electrodes (anodes) is a major obstacle to the development of high-performance RMBs. Mg alloy anodes exhibit better interfacial compatibility with the electrolyte and lower overpotentials during cycling. In this project, we explored the roles of composition, surface structure, and crystal morphology on the interaction of Mg, Mg20Bi, and Mg20Sn alloy anodes with the magnesium borohydride–ammonia solid-state electrolyte Mg(BH₄)₂·2NH₃. We demonstrate that at room temperature, alloying effectively regulates the electrochemical activity of the anode in grain-boundary free-energy driven processes. Moreover, by optimising the crystallographic orientation and grain size of the alloy, its electrochemical performance could be enhanced to accelerate reaction kinetics at both the anode and cathode. The alloy anodes exhibited the better interfacial compatibility with the electrolyte and lower overpotentials during cycling than pure Mg. These results demonstrate that Mg20Bi and Mg20Sn are better alloy anodes for solid-state magnesium batteries at room temperature.