Molten chloride mixtures such as MgCl₂–KCl–NaCl are potential thermal energy storage (TES) materials and heat transfer fluids (HTFs) for next-generation concentrating solar power (CSP) systems due to their high operation temperatures (>700°C) and low costs. This ternary MgCl₂–KCl–NaCl salt mixture with a low material cost (<0.35 $/kg) is a promising salt for high-temperature TES/HTF applications, for example, it could enable next-generation CSP plants to have significant power cycle efficiency and lower levelized cost of electricity (LCOE). In this study, the minimum melting temperature composition for the MgCl₂–KCl–NaCl mixture was estimated by FactSage simulation, while the binary and ternary phase diagrams of the MgCl₂–KCl–NaCl salt system, including their phase transitions, were studied. Five different eutectic compositions of the MgCl₂–KCl–NaCl mixture from literature and simulations were selected to determine the minimum melting temperature composition by the differential scanning calorimetry technique. Results show agreement between experiments and simulations of the melting temperature and its composition. Hence, values of the exact and reliable minimum melting temperature and salt composition in mol% and wt% are given. Based on the determined melting temperature, the minimum operating temperature of MgCl₂–KCl–NaCl is recommended according to the safety margin for CSP applications.