Magnesium-air battery is a promising energy storage device because of its high theoretical voltage, high specific energy, low cost, and greater safety. Due to the passivation of Mg surface with Mg(OH)2 in alkaline electrolyte (pH >11) prevents further reaction of the anode, Mg-air battery typically works in pH-neutral aqueous electrolyte. However, developing electrocatalysts for pH-neutral oxygen reduction with high activity at low cost remains a significant challenge. Here, we report a mixed-phase mullite (SmMn2O5) electrocatalyst as an efficient substitute for Pt/C in pH-neutral oxygen reduction reaction (ORR). The mixed-phase mullite exhibits similar catalytic activity but superior stability to Pt/C in 1M NaCl solutions. Density functional theory simulations reveal that the catalytic activity is correlated to the strength of p-d hybridization between octahedral Mn and lattice O in the mullite structure, which results in a moderate bond strength between adsorbed O atoms and square-pyramidal Mn dimer sites. The presence of CeO2 phase also facilitates the adsorption and diffusion of oxygen atoms on mullite surface. Finally, the feasibility of utilizing mixed-phase mullite catalyst is demonstrated by fabricating Mg-air batteries to power a red light-emitting diode.