MnO₂/carbon nanocomposites with hierarchical pore structure and controllable MnO₂ loading have been synthesized using a self-limiting growth method. This was achieved by the redox reactions of KMnO₄ with sacrificed carbon substrates that contain hierarchical pores. The unique pore structure allows the synthesis of nanocomposites with tunable MnO₂ loading up to 83 wt.%. The specific capacitance of the nanocomposites increased with the MnO₂ loading; the conductivity measured by electrochemical impedance spectroscopy, on the other hand, decreased with increasing MnO₂ loading. Optimization of the MnO₂ loading resulted in nanocomposites with high specific capacitance and excellent rate capability. This work provides important fundamental understanding which will facilitate the design and fabrication of high-performance supercapacitor materials for a large variety of applications.