A new kind of nitrogen-doped graphene (NG) electrocatalyst with well-defined mesoporous structure has been for the first time fabricated by a scalable and templateless technique of directly annealing of pre-synthesized graphene oxide–polydopamine (GO/PDA) nanocomposites. Although with the high-temperature treatment, the obtained NG nanosheets in well-dispersed state are possessed with well-defined mesoporous architecture and large specific surface area owing to the pre-polymerization of dopamine on the GO nanosheets as the spacing. Furthermore, the prepared NG as the electrode material exhibits much more enhanced oxygen reduction reaction (ORR) activities with positive onset potential and large current density than the control samples. Moreover, compared with the commercial Pt/C, the optimized NG electrocatalyst displays better cycling stability and stronger tolerance to methanol crossover effect, making it a promising metal-free methanol–tolerant cathodic catalyst for practical fuel cells.