As they are easily and reproducibly obtained, biomass-derived carbons have received intensive research attention as electrode materials for energy storage devices. In contrast to three-dimensional carbon derived from pomelo peel by traditional KOH processes, two-dimensional carbon derived from pomelo peel is produced through modified processes in this work. The derived lamellar carbon has a large surface area (1927 m2 g−1) and is rich in surface oxygen functional groups. Owing to these advantages, as the electrode of a supercapacitor, it delivers a very high capacitance (398 F g−1 at 1 A g−1) in an aqueous electrolyte and a high energy density (21 Wh kg−1 at a power density of 180 W kg−1) in an ionic liquid. Additionally, it exhibits good cycling stability; after 10,000 cycles, 95% of the capacitance is retained. This good performance makes it a promising electrode for supercapacitors.