Soybean meal or bean pulp is the residual waste obtained after extraction of oil from soybean flakes. This residue has gained significant interests because of its high nitrogen content. In this study, a one-step pyrolysis synchronous activation method has been proposed for preparing nitrogen-doped hierarchical porous bean pulp carbon (BPC) in a CO2 environment. The morphological, structural, and crystallisation properties of the BPC are adjusted by changing the treatment temperature. The BPC coated with MnO2 (BPC@MnO2) nanoparticles are further synthesised in a straightforward manner using a green hydrothermal method. The produced samples are evaluated as electrode materials for supercapacitor application. The results show that the pore structure and nitrogen species of biochar are strongly affected by the presence of CO2 and pyrolysis temperature. The coated MnO2 nanoparticles can be uniformly deposited on the surface of the BPC. The electrochemical performance of the BPC@MnO2 composite is higher than that of the BPC. A specific capacitance of 288.3 F/g can be attained for the BPC@MnO2 at the current density of 0.25 A/g. The symmetrical button capacitor made from the composite successfully lights up the light-emitting diode for more than 300 s. These novel electrode materials derived from bean pulp display considerable potentials for application in energy storage.