The electro-intercalation behavior of Li+ ions has been studied in a solid Mg cathode in LiCl-KCl molten salt. Cyclic voltammetry, chronoamperometry, and chronopotentiometry were employed to investigate the formation behavior of Mg-Li alloys via the electro-intercalation of Li+ ions. The electro-intercalation of Li+ ions into an Mg electrode occurred through underpotential deposition (UPD) to form Mg-Li alloys. The diffusion coefficient of Li+ ions at the Mg cathode was calculated by using Cottrell's equation. The phase of the Mg-Li alloy was determined from the Li content of the alloy. The growth of the alloy layer by electro-intercalation was very fast compared to conventional solid-phase diffusion. The measured Li content of the Mg-Li alloy is similar to the theoretical value calculated on the basis of supplied charges.