Composite materials are synthesized by combining chemically functionalized carbon nano-onions (ONCNOs), a polyelectrolyte (polydiallyldimethylammonium chloride, PDDA) and delaminated layered-manganese oxides (MnO₂). While carbon nano-onions are among the least studied carbon allotropes to date, extensive research has been reported on various types of crystalline and amorphous MnO₂ to overcome their limitations for energy storage applications. We have developed a synergistically tuned synthesis of MnO₂-based composites with ONCNOs via a sequential chemical deposition technique to achieve high-capacity supercapacitor electrodes with long-term stability. A composite with 55 wt% MnO₂ on PDDA modified ONCNOs exhibits a high-capacitance of 218.6 F g⁻¹ in a symmetric two-electrode cell containing an aqueous electrolyte (1.0 M Na₂SO₄) with a high-energy density of 6.14 W h kg⁻¹. Analytical techniques, such as high-resolution transmission electron microscopy, X-ray diffraction, atomic force microscopy, thermogravimetric analysis, and electrochemical measurements (cyclic voltammetry, galvanostatic charge–discharge, and impedance), are employed to fully characterize the synthesized materials and to understand their electrochemical behavior. Emphasis is also given to understand the phenomenon that promotes higher capacitance and long-term stability of such composites based on MnO₂ redox chemistry.