Production cost, capacitance, and electrode materials safety are the key factors to be concerned about for supercapacitors. In this work, a type of carbon nanosheets was produced through the carbonization of tripotassium citrate monohydrate and nitric acidification. Subsequently, a well-designed manganese dioxide/carbon nanosheets composite was synthesized through hydrothermal treating. The carbon nanosheets served as the substrate for growing the manganese dioxide, regulating its distribution, and preventing it from inhomogeneous dimensions and severe agglomeration. Many manganese dioxide nanosheets grew vertically on the numerous functional groups generated on the surface of the carbon nanosheets during acidification. The synergistic combination of carbon nanosheets and manganese dioxide tailors the electrochemical performance of the composite, which benefits from the excellent conductivity and stability of carbon nanosheets. The carbon nanosheets derived from tripotassium citrate monohydrate are conducive to the remarkable performance of manganese dioxide/carbon nanosheets electrode. Finally, an asymmetric supercapacitor with active carbon as the cathode and manganese dioxide/carbon nanosheets as the anode was assembled, achieving an outstanding energy density of 54.68 Wh·kg^–1 and remarkable power density of 6399.2 W·kg^–1 superior to conventional lead-acid batteries. After 10000 charge-discharge cycles, the device retained 75.3% of the initial capacitance, showing good cycle stability. Two assembled asymmetric supercapacitors in series charged for 3 min could power a yellow light emitting diode with an operating voltage of 2 V for 2 min. This study may provide valuable insights for applying carbon materials and manganese dioxide in the energy storage field.