Manganese oxides have attracted great interest as promising pseudocapacitive materials due to its low cost, variable oxidation states, and high theoretical capacitance. In this work, we report the in-situ synthesis of MnO-dispersed carbon nanofibers (CNFs) using electrospinning and a unique two-step carbonization technique. While the single-step carbonization resulted in mixed MnxOy forms, however, by employing a suitable and optimized two-step carbonization, we were able to obtain CNFs assimilated with MnO nanoparticles (MnO-CNFs). The synthesized MnO-CNF electrodes achieved a high specific capacitance of 246 F g−1 at 0.5 A g−1 in a three-electrode system. A symmetric supercapacitor device assembled with these electrodes exhibited remarkable electrochemical performance with a maximum power density of 5000 W kg−1, maximum energy density of 14 W h kg−1, and an excellent cycling stability (97.5% retention after 10,000 cycles). The exceptional electrochemical performance of MnO-CNFs makes them promising electrode materials for practical energy storage applications.