In this study, we explored the correlation between the catalytic activity of UK Biochar Research Center biochars (BCs) and their physicochemical properties, suggesting a potential role of endogenous iron in promoting their persulfate activation capability by heat treatment. A steady improvement in the persulfate activation efficiency with increasing annealing temperature was observed exclusively for iron-containing BCs (e.g., sewage sludge (SS) BCs). Along with increasing the Brunauer–Emmett–Teller surface area, thermal annealing formed carbon-wrapped iron particles on the SS BC surfaces, produced crystalline elemental iron and iron carbides, and caused an sp3-to-sp2 conversion in the carbon phase. Comparing the annealed SS BCs in terms of the open circuit potential shift, electrical conductivity, and thermodynamic binding parameters showed that the heat-initiated modification enhanced the electron transfer-mediating capacity and surface affinity toward persulfate, which led to the beneficial effect of annealing on the carbocatalytic activity of iron-containing BCs for non-radical persulfate activation.