Single-atom Fe–N–C electrocatalysts have demonstrated exceptional selectivity toward CO during CO₂ reduction, yet their practical application is severely hindered by the intrinsic activity–stability trade-off. Herein, we report a straightforward in-situ sulfidation of Fe-doped zeolitic imidazolate frameworks to construct ZnS nanoparticle-modified S-doped Fe–N–C catalysts (ZnS@Fe–NSC). This approach causes the concurrent formation of Fe–N₄ active sites, S doping, and ZnS nanoparticles. Operando characterizations and density functional theory (DFT) calculations reveal that ZnS nanoparticles donate electrons to Fe centers with a 0.5 eV negative shift in Fe 2p binding energy and strengthen Fe–N bonds with an increased integrated crystal orbital Hamiltonian population value from −0.94 to −1.30 eV. This electronic modulation accelerates the formation of the key *COOH intermediate and suppresses the hydrogen-induced Fe leaching by over 20-fold. The ZnS@Fe–NSC exhibits a CO Faradaic efficiency of 98.5% at −0.58 V versus reversible hydrogen electrode and maintains over 90% selectivity for 30 h. When integrated into a Zn–CO₂ battery, it delivers a peak power density of 6.2 mW·cm⁻² and operates stably for 125 h. This work opens an avenue for the rational design of robust single-atom electrocatalysts toward practical CO₂ conversion and beyond.