Mg–air batteries have garnered substantial research attention owing to their high specific capacity, low electrode potential, environmental benignity, and abundant natural magnesium reserves. Nevertheless, sluggish oxygen reduction reaction (ORR) kinetics at the air cathode severely restrict their practical implementation, which necessitates the development of low-cost, high-activity non-precious metal electrocatalysts. Herein, modified Mo₂C@P electrocatalysts are successfully synthesized via phosphidation of Mo₂C mixed with sodium hypophosphite (NaH₂PO₂) under thermal treatment. Multiple characterization techniques are employed to systematically investigate the crystal structure, chemical bonding configuration, micromorphology, and ORR electrocatalytic performance of the as-prepared materials. Meanwhile, Mg–air battery evaluations are carried out in 3.5 wt.% aqueous NaCl electrolyte. The results reveal that the Mo₂C MXene backbone is well retained. Phosphorus doping is confined to surface modification and is accompanied by the formation of P–O and P–C bonds. The stacked lamellar architecture undergoes surface reconstruction, generating a uniform particulate morphology. The ORR electrocatalytic activity exhibits a volcano-type trend with phosphidation temperature, and Mo₂C@P-350 achieves the highest limiting diffusion current density of 2.87 mA cm⁻². The Mg–air battery assembled with Mo₂C@P-350 delivers outstanding and stable discharge voltages of 1.19 V at 5 mA cm⁻² and 1.13 V at 10 mA cm⁻², along with optimal rate capability. This work demonstrates that mild-temperature phosphidation represents a reliable surface-engineering strategy for constructing high-performance ORR electrocatalysts.