A new ternary nanocomposite was prepared by using MnO2, carbon black (CB), and exfoliated graphite (EG) through a sonochemical method. In this process, the MnO2 nanoparticles was anchored on the mixture of CB and EG to maximize the specific capacitances of these materials. Structure and morphology of the CB/EG/MnO2 nanocomposites were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical properties of the CB/EG/MnO2 nanocomposites with different content of MnO2 were studied by cyclic voltammetry (CV), fast Fourier transformation continuous cyclic voltammetry (FFTCCV) technique, galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS). The best nanocomposite electrode displayed specific capacitance of 364 F g−1 at the scan rate of 2 mV s−1 in 0.5 M Na2SO4 aqueous solution, which is higher than pure MnO2 (289 F g−1). The capacitance stability of the nanocomposite electrode was studied by FFTCCV at the scan rate of 500 mV s−1. The result shows that after recording 4000 CVs, the specific capacitance of the nanocomposite decline only 5%. Furthermore, the nanocomposite electrode showed higher energy density than MnO2 electrode.