In this study, a facile synthesis route for preparing manganese dioxide (MnO₂) with unique nano-needle morphology and its nanocomposite with reduced graphene oxide (MnO₂–rGO) for high energy density quasi-solid-state asymmetric supercapacitor (ASC) application is reported. Morphological characterizations indicate that the MnO₂ nano-needles are uniformly decorated on rGO sheets, creating an interconnected structure with rGO. The MnO₂–rGO nanocomposite shows good pseudocapacitive electrochemical behavior in a potential domain from 0 to 1.0 V. The detailed analysis of the cyclic voltammetry (CV) profiles of the rGO and MnO₂–rGO electrodes indicate that their sodium ion storage kinetics are based on ideal capacitive-controlled and pseudocapacitive (capacitive and diffusive) controlled processes, respectively. Furthermore, a quasi-solid-state ASC device is constructed by employing the MnO₂–rGO nanocomposite and rGO as the positive and negative electrodes, respectively. The fabricated ASC (rGO‖MnO₂–rGO) device operates within the wide cell potential range from 0 to 1.8 V and possesses the highest capacitance of 216 F g⁻¹, at the current density of 1 A g⁻¹, which is superior to that of the recently reported literatures. The ASC (rGO‖MnO₂–rGO) device displays an energy density of 24.25 W h kg⁻¹ at corresponding power density of 900 W kg⁻¹ along with significant cycle stability over the 6000 cycles. Thus, the morphological design of an advanced electrode material to boost the capacitance and potential window with the polymer gel electrolyte will aid in the fabrication of high energy density storage devices.