Uniform carbonaceous spheres covered with MnO2 nanoparticles (CMnO2-1) or nanosheets (CMnO2-2) were successfully prepared via a facile in situ seed-assisted method which could accomplish the controlled fabrication of the composites with tunable morphology and mass loading of MnO2. When employed as active materials for supercapacitor electrodes, CMnO2-2 exhibited larger specific capacitance, higher rate capability and better cycling stability than CMnO2-1 due to the fast ion penetration and smaller charge-transfer resistance. It is believed that these results may provide an alternative way to improve the redox kinetics and reversibility of metal oxides-based pseudocapacitors.