Most simple magnesium salts tend to passivate the Mg metal surface too quickly to function as electrolytes for Mg batteries. In the present work, an electroactive salt Mg(THF)6AlCl42 was synthesized and structurally characterized. The Mg electrolyte based on this simple mononuclear salt showed a high Mg cycling efficiency, good anodic stability (2.5 V vs. Mg), and high ionic conductivity (8.5 mS cm−1). Magnesium/sulfur cells employing the as‐prepared electrolyte exhibited good cycling performance over 20 cycles in the range of 0.3–2.6 V, thus indicating an electrochemically reversible conversion of S to MgS without severe passivation of the Mg metal electrode surface. Simple but effective: A simple magnesium salt Mg(THF)6AlCl42 can be used as a magnesium electrolyte that possesses a highly reversible Mg cycling efficiency, good anodic stability, and good ionic conductivity. Mg/S batteries containing the electrolyte could be cycled over 20 cycles, thus indicating electrochemically reversible conversion of sulfur into MgS.