This study demonstrated that metal-carbon composites (Me-N-C; Me = Mn, Fe, Co, Ni, and Cu) featuring sulfidated metal nanoparticles encapsulated within an N-doped carbon matrix activated both persulfate and O₃, albeit through distinct mechanisms. The carbon phase, exhibiting enhanced electrical conductivity due to the presence of internal metal cores, facilitated non-radical persulfate activation. In contrast, the metallic constituent predominantly converted O₃ to hydroxyl radical (•OH). This oxidant-dependent shift in the principal catalytic site (or degradation pathway) was substantiated by a mechanistic examination of oxidant activation by Ni-N-C that varied in structural characteristics and chemical compositions. The persulfate activation capability of Ni-N-C rose proportionally with the content of graphitic-N as the key species in the non-radical activation pathway. Conversely, •OH yield from O₃ correlated strongly with the degree of Ni sulfidation, suggesting that sulfidated Ni functioned as the catalytic center for O₃ activation. The active-site switching was further supported by the impact of H₂-assisted pyrolysis, which suppressed Ni sulfidation while enriching graphitic-N, thereby enhancing persulfate activation but kinetically retarding O₃-to-•OH conversion. UV irradiation, preventing surface organic accumulation, and thermal sulfidation, enriching metal sulfide content, effectively regenerated Ni-N-C by targeting the respective catalytic centers for persulfate and O₃ activation. DFT calculations indicated that Ni3S2 displayed a preferential tendency to dissociatively adsorb and subsequently activate O₃, whereas carbon-encapsulated Ni promoted non-radical persulfate activation at the carbon interface. The identification of oxidant-specific catalytic sites provided a pivotal design rationale for developing metal-carbon composites as versatile catalysts for oxidant activation.