Developing low-cost and highly effective doped carbon catalysts for the oxygen reduction reaction remains an urgent requirement for fuel cell applications. Herein, we design a facile and effective preparation strategy for the fabrication of a 3D porous carbon network catalyst comprised of an ultrathin nanosheet and anchored with Fe3C nanoparticles. The catalyst was prepared using an iron–tannin framework coated over g-C3N4 as precursor, and simultaneously with g-C3N4 as a nitriding agent and structural/morphological template. Optimum catalyst exhibits excellent ORR performance and durability in an alkaline medium; the half-wave potential (+0.86 V vs. RHE) is 40 mV more positive than that of commercial Pt/C, and its current density at +0.9 V (vs. RHE) reaches −1.153 mA cm−2, which is almost 2.42 times that of commercial Pt/C. Significantly, the catalyst also shows outstanding ORR performance in acidic conditions with a half-wave potential of +0.73 V (vs. RHE), comparable to that of Pt/C, and better long-term stability than Pt/C. Based on our characterization results, we ascribe the outstanding performance of catalyst to: the enhanced amount of Fe–Nx active sites and active nitrogen species, including pyridinic-N and graphitic-N; Fe3C nanoparticles covered with ultrathin doped carbon layer; and the high surface area and porous structure.