Intermetallic InBi has been prepared for the first time by high-energy mechanochemical synthesis from metallic In and Bi powders. Considering that both In and Bi are able to electrochemically alloy with Mg, InBi-based electrodes in Mg-battery were also investigated by operando X-ray diffraction in order to verify a possible synergy between the two metals in the reaction vs. Mg. In contrast to the relatively simple magnesiation mechanisms of both pure metals, the overall alloying mechanism of InBi – leading to the formation of Mg3Bi2 and MgIn – reveals an unexpected complex pathway through several successive intermediate phases, with InBi being reformed as a major product at the end of the charge. The presence of lone In during the reversible magnesiation process strongly affects the electrochemical properties. Indeed the cycling performance is intermediate between those of pure In- and Bi-based electrodes, without any beneficial synergy between the two active elements.