AbstractDevelopment of practical rechargeable Mg batteries (RMBs) is impeded by their limited cycle life and rate performance of cathodes. As demonstrated herein, a copper‐porphyrin with meso‐functionalized ethynyl groups is capable of reversible two‐ and four‐electron storage at an extremely fast rate (tested up to 53 C). The reversible four‐electron redox process with cationic‐anionic contributions resulted in a specific discharge capacity of 155 mAh g−1 at the high current density of 1000 mA g−1. Even at 4000 mA g−1, it still delivered >70 mAh g−1 after 500 cycles, corresponding to an energy density of >92 Wh kg−1 at a high power of >5100 W kg−1. The ability to provide such high‐rate performance and long‐life opens the way to the development of practical cathodes for multivalent metal batteries.