The demand for energy storage systems with superior performance has led to the creation of hybrid supercapacitor device. With proper designs, the hybrid supercapacitive materials can achieve high performance while reducing the overall cost. Herein, a novel method is developed for preparing three-dimensional hierarchical graphitic carbon nanocomposites with highly dispersed mixed Co-Ni oxide nanoparticles (Co-Ni-O/3DG) by a facile one-pot process involving carbonization and subsequent oxidation of metal ion doped biopolymer precursors. The mixed metal nanoparticles produced during carbonization enabled a top-down preparation of 3D graphitic carbon nanosheets. The nanocomposites were fully characterized and showed excellent electrochemical performance supported by the DFT calculation. Specific capacitance of 1586 F·g ^-1 was achieved (current density 1.0 A·g ^-1 ), with capacitance retention of 94.5% after 10 000 cycles demonstrating exceptional cycling stability. In an asymmetric full-cell system using a Co-Ni-O/3DG positive electrode, high energy densities of 32.8-54.7 Wh kg ^-1 associated with very high power densities of 11358-748.6 W kg ^-1 were obtained, comparable to the most advanced contemporary supercapacitive materials, while they also possesses an improved cyclability as well as using biosourced staring materials, underlining the electrode's potential application in hybrid supercapacitor devices.