Graphitic carbon nitrides (g-C₃N₄) possess various benefits as heterogeneous photocatalysts, including tunable bandgaps, scalability, and chemical robustness. However, their efficacy and ongoing advancement are hindered by challenges like limited charge-carrier separation rates, insufficient driving force for photocatalysis, small specific surface area, and inadequate absorption of visible light. In this study, boron dopants and nitrogen defects synergy are introduced into bulk g-C₃N₄ through the calcination of a blend of nitrogen-defective g-C₃N₄ and NaBH₄ under inert conditions, resulting in the formation of BCN nanosheets characterized by abundant porosity and increased specific surface area. These BCN nanosheets promote intermolecular single electron transfer to the radical initiator, maintaining radical intermediates at a low concentration for better control of photoinduced atom transfer radical polymerization (photo-ATRP). Consequently, this method yields polymers with low dispersity and tailorable molecular weights under mild blue light illumination, outperforming previous reports on bulk g-C₃N₄. The heterogeneity of BCN enables easy separation and efficient reuse in subsequent polymerization processes. This study effectively showcases a simple method to alter the electronic and band structures of g-C₃N₄ with simultaneously introducing dopants and defects, leading to high-performance photo-ATRP and providing valuable insights for designing efficient photocatalytic systems for solar energy harvesting.