Manganese oxide and carbon hybrid materials have attracted intensive research attention as advanced electrode materials for energy storage. Herein, starting form metal-organic frameworks (MOFs), the nanocomposites of mixed-valent manganese oxide nanoparticles in-situ grown on ultrathin carbon sheets (MnO x -CSs nanocomposites) have been synthesized via a simple method. Benefiting from the unique structural merits, the nanocomposite obtained at 600°C (MnO x -CSs-600) exhibits excellent performance for both supercapacitors (SCs) and lithium-ion batteries (LIBs). A high specific capacitance of 220Fg−1 is achieved at the current density of 1Ag−1. An asymmetric supercapacitor (ASC) based on the MnO x -CSs-600 nanocomposite cathode and activated carbon anode exhibits a high energy density of 27.5Whkg−1 at the power density of 225Wkg−1. Moreover, the MnO x -CSs-600 nanocomposite displays a high reversible capacity of 1217.7mAhg−1 at 200mAg−1 after 160 cycles as an anode material for LIBs. Remarkably, the discharge capacity is still as high as 612.1mAhg−1 even at high current density of 2000mAg−1, indicating good rate capability. This synthetic strategy is quite simple, cost-effective and environmental friendly, which is highly promising for scaled-up production.