Sustainable biomass-derived carbons in powdery forms have shown to be high-performance capacitive electrode materials for application in supercapacitors (SCs) due to their large specific surface area, distinct porous structure, and low cost; however, their practical applications are being largely hampered due to their powdery status-related issues including tedious electrode assembly process and insufficient capacitance and cycling stability. Herein, we develop a porous carbon membrane, termed as PECM, by direct carbonization of Pleurotus eryngii (PE) using KOH as activator. The as-fabricated PECM features a hierarchically nanostructure with high porosity, large specific surface area, and excellent electrolyte wettability, which cannot only afford a large usable active area for the interfacial adsorption of electrolyte ions but also supply fast diffusion pathways for electrolyte ions. Moreover, the as-fabricated PECM is doped with N and also has large amounts of O, S, and P-containing functional groups, which would be expected to contribute additional pseudocapacitance. More importantly, the as-fabricated PECM has excellent mechanical performance and can be directly used to assembly an aqueous symmetric SC device. Attributing to the above structural merits of PECM, the two-electrode symmetric SC assembled using two identical PECM-800 electrodes (~20 mg cm⁻² for each electrode), exhibits excellent electrochemical performance with ultrahigh areal capacitance (4.6 F cm⁻² at 10 mA cm⁻²), excellent long-term cycling stability (113% capacitance retention after 20 000 cycles), and superior energy densities of 0.24 to 0.09 mWh cm⁻² at power densities of 5.20 to 51.9 mW cm⁻². This work offers a new insight on employing biomass to prepare value-added and practically applicable carbon materials for the application in SCs.