Sheet-like graphitic carbon with a porous structure can provide low-resistant pathways and short ion-diffusion channels for energy storage, and thus is expected to be an excellent material for high-power supercapacitors. Herein, porous graphene-like nanosheets (PGNSs) with a large surface area were synthesized for the first time via an easy and cost-effective SAG (simultaneous activation–graphitization) route from renewable biomass waste coconut shell. In the synthesis, the graphitic catalyst precursor (FeCl3) and activating agent (ZnCl2) were introduced simultaneously into the skeleton of the coconut shell through coordination of the metal precursor with the functional groups in the coconut shell, thus making simultaneous realization of activation and graphitization of the carbon source under heat treatment. Notably, the iron catalyst in the framework of the coconut shell can generate a carburized phase which plays a key role in the formation of a graphene-like structure during the pyrolytic process. Our results indicated that PGNSs possess good electrical conductivity due to the high graphitic degree, exceptionally high Brunauer–Emmett–Teller surface area (SBET = 1874 m2 g−1) and large pore volume (1.21 cm3 g−1). While being used as a supercapacitor electrode without the use of any conductive additives, PGNSs exhibit a high specific capacitance of 268 F g−1, much higher than that of activated carbon (210 F g−1) fabricated by only activation and graphitic carbon (117 F g−1) by only graphitization at 1 A g−1. Also, PGNSs show superior cycle durability and Coulombic efficiency over 99.5% after 5000 cycles in KOH. Remarkably, in an organic electrolyte, PGNSs also display an outstanding capacitance of 196 F g−1 at 1 A g−1. An energy density of up to 54.7 W h kg−1 could be achieved at a high power density of 10 kW kg−1. The SAG strategy developed here would provide a novel route for low-cost and large-scale production of PGNS electrode materials for high-power supercapacitors.