Cobalt nanoparticles (NPs) immobilized on N-doped carbonaceous substrates are attractive heterogeneous catalysts for activating Oxone to degrade pollutants. However, conventional preparation of Co NPs/N-doped carbon composites involves multi-step syntheses of N-doped carbon substrates, and then immobilization of Co NPs on substrates, which are complicated and time-consuming. In this study, a convenient one-step fabrication technique is developed for preparing a composite of Co/N-doped carbon via carbonization of a mixture of melamine and cobalt acetate. The resulting Co@CN is comprised of Co NPs evenly distributed over a carbon nitride (CN) matrix. Co@CN could exhibit porous structures and magnetic controllability, making it an appealing catalyst for Ozone activation. Catalytic activities for Oxone activation by Co@CN are investigated via batch-type decolorization experiments of amaranth (AMR) dye. Co@CN shows much higher catalytic activities than C3N4 and Co3O4, the benchmark catalyst, for Oxone activation to decolorize AMR. In comparison to the other reported catalysts, Co@CN demonstrates the much lower activation energy for AMR decolorization. The mechanism of AMR decolorization by Co@CN-activated Oxone is also determined by investigating Electron paramagnetic resonance (EPR) analysis and effects of radical inhibitors on AMR decolorization. These comparisons indicate the promising features of Co@CN as a heterogeneous catalyst for activating Oxone. The fabrication technique proposed here can be also adopted to develop similar composites of metallic NPs distributed over CN matrices for various catalytic applications.