We prepare electrospun nanofibers (NFs) from a poly(N-vinylpyrrolidone)/manganese (II) acetate composite by electrospinning for application in supercapacitors. As-spun hybrid NFs are calcined at different temperatures to remove the polymer matrix, resulting in inorganic MnO x NFs of variable compositional ratios of Mn3O4 to Mn2O3. Interestingly, the sample of MnO x NFs that is calcined at 500 °C exhibits the highest electrochemical performance, reaching a high specific capacitance of 360.7 F g−1 at a current density of 1 A g−1. This remarkable performance is attributed to the unique three-dimensional morphology and the reduced interfacial resistance of the mixed-phase MnO x NFs within the matrix electrode and the electrolyte.