AbstractA key challenge for practical magnesium–sulfur (Mg–S) batteries is to overcome the sluggish conversion kinetics of sulfur cathodes, achieving a high energy density and long‐lasting battery life. To address this issue, a doping strategy is demonstrated in a model Ketjenblack sulfur (KBS) cathode by introducing selenium with a high electronic conductivity. This leads to a significantly enhanced charge transfer in the resultant KBS_1−xSe_x cathodes, giving rise to a higher S utilization and less polysulfide dissolution. Compared to the bare S cathode, the S‐Se composite cathodes exhibit a higher capacity, smaller overpotentials, and improved efficiency, serving as better benchmark compounds for high‐performance Mg–S batteries. First principles calculations reveal a charge transport mechanism via electron polaron diffusion in the redox end‐products, that enhances the reaction kinetics. By suppressing polysulfide dissolution in the electrolyte, the use of the KBS_1−xSe_x cathodes also enables a more uniform anode reaction, and thereby significantly extends the cyclability of the cells. To improve the performance, further efforts are made by implementing a Mo₆S₈ modified separator into the cell. With an optimized cathode composition of KBS_0.86Se_0.14, the cell applying modified separator shows an improvement of capacity retention by >50% after 200 cycles.