A nickel-manganese layered double hydroxide/graphitic carbon nitride sheets nanocomposite was synthesized via a hydrothermal method with systematically varied graphitic carbon nitride sheets-to-nickel-manganese layered double hydroxide ratios to optimize electrochemical performance. Comprehensive characterization techniques, including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and Brunauer–Emmett–Teller, were employed to elucidate the structural, morphological, and surface properties of the synthesized materials. Electrochemical evaluations, conducted using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy, demonstrated that the composite with 50 mg GCNS exhibited superior capacitive behaviour. In a three-electrode configuration, NMG50 delivered a high specific capacitance of 1523 F g−1 at 1 A g−1 and retained 93% of its initial capacitance after 5000 cycles. In a two-electrode asymmetric configuration, NMG50//activated carbon achieved an energy density of 54.6 Wh kg−1 and a power density of 648 W kg−1, along with 90% capacitance retention over 10,000 cycles. The enhanced performance is attributed to the synergistic interaction between GCNS and NM, which promotes uniform dispersion of LDH nanosheets, increases the availability of electroactive sites, and facilitates efficient charge transport. These findings underscore the potential of NM/GCNS nanocomposites as promising electrode materials for next-generation high-performance supercapacitor applications.