<p>In the present work, a magnesium-intercalated birnessite MnO₂ nanocomposite has been synthesized using the co-precipitation method at room temperature and utilized as an advanced cathode material for hybrid supercapacitor application. The morphology, elemental analysis, and crystalline form of the MgMnO₂ nanocomposite were investigated using various techniques such as scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. XRD patterns showed that the diffraction peaks of MgMnO₂ shift slightly to a lower angle, indicating Mg²⁺ intercalation in the layers of pristine birnessite MnO₂. FTIR spectra of the MgMnO₂ showed an absorption peak at 863&#xa0;cm⁻¹ which is not visible in the pristine birnessite MnO₂ indicating the presence of Mg²⁺. EDX analysis confirmed the presence of Mg²⁺ in the composite, although no significant change in the morphology was observed in the SEM images of pristine birnessite MnO₂ and MgMnO₂ nanocomposite. The electrochemical properties of the nanocomposite were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge behavior of the nanocomposite. The results have demonstrated a specific capacitance of 363&#xa0;F&#xa0;g⁻¹ at a current density of 1&#xa0;A&#xa0;g⁻¹ for MgMnO₂ nanocomposite with excellent charge/discharge properties, where 94% of its specific capacitance is retained after 5000 cycles at a current density of 10&#xa0;A&#xa0;g⁻¹. Hence, the nanocomposites synthesized and presented in the present work demonstrate huge potential for their use as cathode material for hybrid supercapacitor application.</p>