Experimentally synthesized Fe-N-C catalysts invariably exhibit mixed Fe spin states, making it impossible to distinguish the individual contributions of different spin configurations to oxygen reduction reaction (ORR) activity. This has led to conflicting reports, while conventional simulations fail to capture the ORR mechanism under such complexity, creating a significant knowledge gap. Here, we uncover a “spin state locking” mechanism that resolves this puzzle. Using constant-potential ab initio molecular dynamics, we demonstrate that adsorbed H₂O kinetically locks the Fe spin state throughout the ORR cycle, preventing spin crossover and confirming that multiple spin states actively contribute to catalysis. By directly comparing kinetics across spin states, we find that high-spin (HS) Fe(II) exhibits superior activity with a half-wave potential of 0.73 V (vs. standard hydrogen electrode (SHE)), far exceeding that of intermediate-spin (IS) FeN₄ (0.46 V). Surprisingly, although OH-coordinated Fe(III) is thermodynamically predicted to be highly active, its kinetic barrier for O₂ activation results in a poor half-wave potential of only −0.1 V. Our work not only establishes the crucial role of spin state locking in Fe-N-C ORR mechanism but also provides the first unambiguous differentiation of how each spin component contributes in mixed-spin catalysts. These insights fundamentally advance the understanding of spin-dependent electrocatalysis.