Non‐nucleophilic electrolytes that can reversibly plate/strip Mg are essential for realizing high‐performance rechargeable Mg/S batteries. In contrast to organometallic electrolytes, all‐inorganic electrolytes based on MgCl2‐AlCl3 complexes are more cost‐effective and hold better stability to air and moisture. A recently developed electrolyte that contains tetrahydrofuran solvated divalent Mg cation, Mg·6THFAlCl42, has exhibited decent compatibility with the sulfur cathode. However, it suffers a large overpotential and short cycle life, which hinders its applications in Mg/S batteries. Here, an efficient plating/stripping of Mg is realized successfully by using LiCl to dissolve MgCl2 from the electrolyte/electrode interface. As a result, the overpotential of Mg plating/stripping is remarkably reduced to 140/140 mV at a current density of 500 µA cm−2. Both experiments and density functional theory (DFT) calculations reveal that the LiCl‐assisted solubilization of MgCl2 facilitates the exposure of fresh surface on the Mg anode. Utilizing such an LiCl‐activation strategy, Mg/S full batteries with a significantly extended cycle life of over 500 cycles, as well as coulombic efficiency close to 100%, are achieved successfully. This work demonstrates the role of LiCl‐assisted interface activation on extending the cycle‐life Mg/S batteries with all‐inorganic electrolytes. The electrolyte–electrode interface of an Mg anode could be passivated by the formation of low‐solubility species such as MgCl2. Here, it is found that an LiCl additive could effectively boost the interfacial property by forming soluble intermediate species such as Mg2(µ‐Cl)3·6THF+ and LiCl2·2THF−, which is critical to sustain a fresh Mg surface for Mg/S batteries with long cycle life.