Biochar loading mixed-phase iron oxide shows great advantages as a promising catalyst owing to its eco-friendliness and low cost. Here, γ-FeO@biochar (E/Fe-N-BC) composite was successfully prepared by the sol-gel method combined with low-temperature (280 °C) reduction. The Scanning Electron Microscope (SEM) result indicated that γ-FeO particles with the size of approximately 200 nm were well-dispersed on the surface of biochar. The CO derived from biomass pyrolysis is the main reducing component for the generation of Fe (II). The high content of Fe (II) contributed to the excellent catalytic performance of E/Fe-N-BC for quinclorac (QNC) degradation in the presence of peroxymonosulfate (PMS). The removal efficiency of 10 mg/L of QNC was 100% within 30 min using 0.3 g/L γ-FeO@biochar catalyst and 0.8 mM PMS. The radical quenching experiments and electron paramagnetic resonance analysis confirmed that •OH and SO• were the main radicals during the degradation of QNC. The facile and easily mass-production of γ-FeO@biochar with high catalytic activity make it a promising catalyst to activate PMS for the removal of organic pollutants.