The electrochemical behavior of MgCl2 was investigated in ternary NaCl-KCl-CaCl2 molten chloride mixtures, using inert tungsten and steel electrodes in the temperature range from 973 to 1053 K. Electrochemical methods like square wave voltammetry (SWV), cyclic voltammetry (CV), chronoamperometry (CA), and chronopotentiometry (CP) were applied to investigate the reaction process. Results showed that reduction of Mg2+ to Mg was not a reversible process in the system. The reduction of magnesium ion on tungsten electrodes was a single-step process with the exchange of two electrons calculated by the methods of SWV and CV. The nucleation mechanism of magnesium deposition on a tungsten substrate was studied according to the electrochemical model of Scharifker-Hill, indicating that nucleation process of magnesium on the tungsten electrode was instantaneous nucleation, which was consistent with the formation and growth mechanisms of hemispherical nuclei. The diffusion coefficients of Mg2+ calculated by different methods of CV, CA, and CP at 973 K were 1.44 × 10−5, 1.75 × 10−5, and 1.27 × 10−5 cm2 s−1. In addition, the Arrhenius treatment was employed to obtain the activation energy for the diffusion process in the molten salt mixtures. The activation energy values for diffusion of Mg2+ were derived using CV and CA electrochemical techniques as Ea = 47.89 and 53.3 kJ mol−1, respectively. The deposition of Mg metal was identified by SEM-EDS and XRD analysis method.