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Transition-metal (TM)-based spinel oxides have demonstrated excellent efficacy in Fenton-like reactions, but the key mechanism behind peroxymonosulfate (PMS) adsorption, decomposition, and pollutant degradation is still unclear. Here, a crucial role of eg orbital occupancy in manipulating the interaction between PMS and Zn–Co–Mn ternary spinel catalysts and the resulting pollutant degradation is first discovered. The introduction of Co into the ZnMn2O4 network lowers the magnetic momentum and eg occupancy and favors the overlap between TM eg and O 2p orbitals. Experimental results demonstrate that the eg occupancy-dependent catalytic activity and pathway originate from its cascade effect on PMS binding, decomposition, and radical desorption. Zn–Co–Mn with optimized eg occupancy exhibits favorable PMS binding strength, interaction capability, radical desorption, and pollutant degradation. Cyclic voltammetry (CV) and density functional theory (DFT) corroborate the critical role of eg in PMS affinity. In addition, the ZnCoMnO4/PMS system shows high selectivity for carbamazepine (CBZ, 0.275 min–1) and environmental robustness. The surface active complex PMS*, the peroxymonosulfate radical, and the sulfate radical are identified as reactive species. This work provides an intrinsic mechanism behind pollutant degradation and offers guidance for performance enhancement in a water environment.