A novel MnO petal nanosheet@carbon sphere core-shell structure was successfully fabricated by adjusting the quantity of the KMnO precursor employed during the in situ growth of MnO on the surface of carbon spheres via a facile hydrothermal method. In the presence of low KMnO contents, only MnOOH was generated. In contrast, upon increasing the quantity of KMnO, δ-MnO nanorods and petal nanosheets were obtained, thereby allowing the formation of the core-shell structured δ-MnO petal nanosheet@carbon sphere composites. However, beyond a certain point, further increases in the KMnO content were unfavorable. Although, prepared MnO/carbon sphere composites of different morphologies can be used for supercapacitors (SCs) electrode materials, we found that the core-shell structured MnO petal nanosheet@carbon sphere composites exhibited the optimal capacitance performances in all the composites. These composites exhibited an excellent specific capacitance of 231 F⋅g under a current density of 0.5 A⋅g. Furthermore, they also demonstrated an impressive cycling stability. Indeed, after 1,000 cycles at 10 A⋅g, the MnO petal nanosheet@carbon spheres exhibited 96% of their initial capacitance in a 1 M NaSO aqueous electrolyte. The synergistic effect between δ-MnO and the porous carbon spheres in the unique core-shell structured is responsible for the excellent cycle life.