 A simple, facile, cost-effective and reproducible one-pot sonochemical synthesis of hydroxy-functionalized graphene–MnO2 nanocomposite of various compositions is executed starting from pristine graphite powder. Morphology of all the as-synthesized composites has been characterized with scanning electron microscopy and high-resolution transmission electron microscopy. Further, X-ray diffraction, Thermogravimetric analyses, Fourier transform-infrared spectroscopy, and Raman spectroscopy have been employed to understand the existing phases, thermal stability, nature of functionalization, and existing interactions between the components, respectively, in all the nanocomposites studied. The as-synthesized sample (GM2) having molar ratio of C:Mn of 0.931 exhibits an improved and synergistic capacitive behavior. The crystalline MnO2 nanosheets anchored on hydroxy-functionalized graphene (GM2) displayed superior specific capacitance of 376.7 Fg−1 as determined by cyclic voltammetry, carried out at a scan rate of 2 mVs−1 in the voltage window of −0.2 to +0.4 V versus SCE in 1 M Na2SO4 medium. It also displayed appreciable capacity retention (~90%) even after 1000 cycles, observed from galvanostatic charging/discharging measurements. Moreover, electrochemical impedance spectroscopic studies of the GM2 sample substantiate better electrical conductivity and ion transport performances, assuring its potentiality as a promising electrode material for energy storage devices.