Interactions between metals and heteroatom-coordinated sites on the carbon matrix are crucial for enhancing the kinetics and thermodynamics of Mg/MgH₂. Herein, we reported a sustainable K₂FeO₄ activation strategy that converts high-sulfur petroleum coke into a sulfur self-doped porous carbon hosting FeNi–S coordinated active sites. Tailoring the alloyed electronic structure and exposing more catalytically active sites substantially enhanced the hydrogenation and dehydrogenation kinetics of Mg/MgH₂ with (FeNi)S@PPC. Its peak dehydrogenation temperature was 95.39 ℃ lower than that of ball-milled MgH₂. In addition, it enabled MgH₂ to release 4.89 wt.% H₂ within 20 min at 275 ℃, exceeding its sulfur-free counterpart (FeNi)@PPC by 1.41 wt.%. Moreover, the (FeNi)S@PPC/MgH₂ showed 99.5% capacity retention after 30 cycles, indicating excellent reversibility. Mechanistic investigations revealed that intrinsic sulfur self-doping induced a stable FeNi–S coordination environment, which lowered the D-band center of FeNi to −1.349 eV. The electronic redistribution was found to weaken the FeNi–(H) intermediate, lowering the dissociation and diffusion energy barrier by 0.39 eV and facilitating hydrogenation. This also reduced the energy required for Mg–H dissociation into H₂ by 0.65 eV. Consequently, the dehydrogenation activation energy was decreased to 97.09 kJ·mol⁻¹, with the rate-limiting step shifting to a low-energy barrier three-dimensional interfacial reaction (R3 model). Overall, this study establishes a green valorization route for high-sulfur petroleum coke and elucidates a fundamental metal–sulfur charge transfer mechanism that substantially enhances magnesium-based hydrogen storage.