Despite recent remarkable progress associated with the electrolyte, understanding of the reaction mechanism of magnesium-sulphur batteries is not yet mature. In particular, the lethargic redox reactions involved in the electrochemical conversion of sulphur and MgS in the cathode need to be overcome. Here, we unveil the reaction mechanism involving copper (Cu) metal, a common current collector for electrodes in rechargeable batteries. Specifically, Cu can undergo chemical reactions with polysulphides produced from the reaction of sulphur or MgS with Mg2+. Throughout the conversion reaction, these Cu-polysulphide reactions play a critical role to improve reaction kinetics markedly. The present investigation opens new avenues to the emerging Mg-S battery technology, that is, the incorporation of various metals that can speed up the conversion reaction between sulphur and Mg.