A 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$_{1−x}$Se$_x$ 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$_{1−x}$Se$_x$ 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 Mo$_6$S$_8$ modified separator into the cell. With an optimized cathode composition of KBS$_{0.86}$Se$_{0.14}$, the cell applying modified separator shows an improvement of capacity retention by >50% after 200 cycles.