MnOx/sewage sludge-derived activated carbon (MnOx/SAC) was prepared as catalysts to improve the performance of aqueous oxalic acid degradation by ozonation. The results indicated that MnOx/SAC had excellent catalytic activity in mineralization of oxalic acid during heterogeneous catalytic ozonation process. MnOx/SAC with a manganese load of 30% exhibited the strongest catalytic activity under the condition of solution pH3.5, which enhanced the oxalic acid removal from 10.3% to 92.2% in 60min compared with that treated by ozone alone. Increase of catalyst dosage and aqueous ozone concentration was advantageous for oxalic acid removal from water. On the basis of catalyst characterization analysis and the observation of inhibitory effect induced by higher pH, less catalyst dosage as well as the presence of hydroxyl radical scavenger, it was deduced that the reaction mechanism involved both hydroxyl radicals attack and surface reactions. Display omitted •Sewage sludge was converted into catalyst (MnOx/SAC) in ozonation and characterized.•The effectiveness of mineralization of oxalic acid by catalytic ozonation with MnOx/SAC was proved under various conditions.•The enhancement of catalytic activity was attributed to surface reactions and hydroxyl radical generation.•The removal rate of oxalic acid had a positive relationship with the surface hydroxyl group in the catalyst.