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Transition metal supported N-doped carbon (M-N-C) catalysts for oxygen reduction reaction (ORR) are viewed as the promising candidate to replace Pt-group metal (PGM) for proton exchange membrane fuel cells (PEMFCs). However, the stability of M-N-C is extremely challenging due to the demetalation, H2O2 attack, etc. in the strongly oxidative conditions of PEMFCs. In this study, we demonstrate the universal effect of Zn on promoting the stability of atomically dispersed M-Nx/C (M = Co, Fe, Mn) catalysts and the enhancement mechanism is unveiled for the first time. The best-performing dual-metal-site Zn-Co-N-C catalyst exhibits a high half-wave potential (E1/2) value of 0.81 V vs. reversible hydrogen electrode (RHE) in acid and outstanding durability with no activity decay after 15,000 accelerated degradation test (ADT) cycles at 60 °C, surpassing most reported Co-based PGM-free catalysts in acid media. For comparison, the Co-N-C in the absence of Zn suffers from a rapid degradation after ADT due to the demetalation and higher H2O2 yield. X-ray adsorption spectroscopy (XAS) and density functional theory (DFT) calculations suggest the more negative formation energy (by 1.2 eV) and increased charge transfer of Zn-Co dual-site structure compared to Co-N-C could strength the Co-N bonds against the demetalation and the optimized d-band center accounts for the improved ORR kinetics. [ABSTRACT FROM AUTHOR]