A novel ternary composite of sulfonated graphene/MnO2/polyaniline (sGMOPANI) is synthesized via a dilute in-situ polymerization method using potassium permanganate as oxidant under neutral condition. The FE-SEM and HRTEM images show that MnO2/polyaniline nanoparticles with the diameter of 5–10 nm deposit onto sulfonated graphene nanosheets. Spectral analysis reveals the existence of the hydrogen bond and π–π interaction between MnO2/polyaniline nanoparticles and sulfonated graphene. As a result, the sGMOPANI composite based on the two-electrode cell shows improved electrochemical capacitance (276 F g−1 at 1 g−1 in 1 M Na2SO4), better rate capability (73% capacitance retention from 0.2 to 20 A g−1), and higher cycling stability (11.7% capacitance loss after 3000 cycles) compared to the MnO2/polyaniline binary composite.