A new approach to fabricate carbon nanosheets-supported MnO2 nanosheet arrays (MnO2/CNS) from Mn-MOF nanosheet-derived MnO/CNS through a facile low-temperature oxidation under a strong alkaline condition has been developed. This low-temperature approach to obtain the high-valence Mn species from low-valence Mn species avoids the oxidation of carbon during the conventional high-temperature synthesis. The as-prepared MnO2/CNS exhibits excellent electrochemical performance as an electrode material for supercapacitors with a maximum specific capacitance of 339 F g(-1) at 0.5 A g(-1) in 1 M Na2SO4 aqueous electrolyte. In addition, the MnO2/CNS displays ultrahigh stability with capacitance retention of 96.1% after 5000 charge-discharge cycles at a current density of 5 A g(-1), which is superior than most of the previously reported MnO2/carbon materials. The excellent capacity and cycling stability of MnO2/CNS are mainly due to its large specific surface area and strong coupling between the in situ formed MnO2 nanosheet arrays and carbon nanosheets. This work provides a new low-temperature approach for the synthesis of high-valence metal oxides/carbon composites from MOF-derived materials for the applications such as energy storage and conversion.