Heteroatoms (B, N, O)-containing porous manganese oxide (MnO2)/carbon nanofiber (MnB-CNF) materials are prepared by one-step electrospinning method via polyacrylonitrile (PAN) and manganese(II) chloride (MnCl2) in dimethylformamide (DMF) solution containing different concentrations of B2O3. The MnB-CNF electrode exhibits optimized electrochemical behavior with a high energy density of 22.6Whkg−1 at a power density of 400Wkg−1 and a specific capacitance range of 210–160Fg−1 in the discharge current density range of 1.0 to 20mAcm−2 in aqueous KOH electrolyte. The higher electrochemical performance of MnB-CNF as a result of the electrochemical double-layer capacitor (EDLC), compared to regular Mn-CNF without B-based functional groups, is attributed to well-balanced meso- and micropores affecting the easy adsorption and transport of electrolyte ions, in addition to the pseudocapacitive redox reactions from MnO2, N, O, and extra numerous B in alkaline electrolytes. Thus, tailoring the pore structures with proper specific surface area, pore size, and number of heteroatoms is crucial for optimizing their electrochemical properties in the combined efforts to develop EDLCs and pseudocapacitance.