Hierarchical nitrogen-doped porous carbons (HNPCs) with tunable pore structures and ultrahigh specific surface areas were designed and prepared from sustainable biomass precursor cellulose carbamate via simultaneous carbonization and activation by a facile one-pot approach. The as-synthesized HNPCs exhibited an ultrahigh specific surface area (3700 m² g⁻¹), a high pore volume (3.60 cm³ g⁻¹) and a high level of nitrogen-doping (7.7%). The HNPCs were structurally tunable in terms of their pore structure and morphology by adjusting the calcination temperature. In three-electrode systems, the electrode made of HNPCs prepared at 900 °C (HNPCs-900) showed a high specific capacitance of 339 F g⁻¹ in 6 M KOH aqueous electrolyte and 282 F g⁻¹ in 1 M H₂SO₄ electrolyte at a current density of 0.5 A g⁻¹. An outstanding rate capability (∼73% retention at a current density of 20 A g⁻¹) and excellent cycling stability (∼95% retention after 5000 galvanostatic charge–discharge cycles at a current density of 5 A g⁻¹) in KOH electrolyte were achieved. In two-electrode systems, the electrode made of HNPCs-900 exhibited a high specific capacitance of 289 F g⁻¹ at 0.5 A g⁻¹ and good rate capacity (∼72% retention at a current density of 20 A g⁻¹) as well as cycling stability (∼92% retention at 2 A g⁻¹) after 5000 cycles. Furthermore, the HNPCs-900 showed an unprecedented adsorption capacity for methylene blue (1551 mg g⁻¹) which was among the few highest ever reported for dye removal. The HNPCs could be used as functional materials for energy storage and waste water treatment.