Ferrous ions (Fe2+) are environmentally friendly materials but show extremely inefficient persulfate activation. Polymeric graphitic carbon nitride (g-C3N4) has recently shown potential to activate persulfates, but the process requires intense light irradiation. To overcome these drawbacks, we designed an innovative heterogeneous iron catalyst by doping Fe into g-C3N4 (Fe-g-C3N4) and used it to activate peroxymonosulfate (PMS) for degradation of pollutant phenol. The catalysts synthesized were fully characterized with various techniques, such as X-ray diffraction, Mossbauer spectroscopy, and X-ray photoelectron spectroscopy. Fe was found to be coordinated with the framework of g-C3N4. Approximately 100% degradation of phenol was achieved with Fe-g-C3N4 after 20 min of reaction, whereas less than 5% degradation of phenol was achieved with Fe2+. Fe-g-C3N4-PMS had a wide effective pH range, and its reactivity was nearly independent of natural illumination. In contrast to the previously proposed radical mechanisms, quenching experiments revealed that nonradical oxidation contributed to the observed degradation. The O-O bond in the activated PMS likely underwent heterolysis, producing high-valence iron species (Fe-IV=O) as the primary active species. These findings have important implications for the development of a selective heterogeneous nonradical-oxidation process.