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Ultrahigh power is needed beyond the state-of-the-art supercapacitors for emerging technologies. Towards this goal, nanostructured carbon materials have been extensively studied but unfortunately encountered with the difficulty in practical applications, which is largely due to their limited packing densities. In this work, we utilize a highly dense commercial activated carbon to develop high-rate nanocomposite electrodes and ultrahigh-power supercapacitors by compositing activated carbon with carbon nanomaterials, including zero-dimensional carbon black, one-dimensional carbon nanotube, and two-dimensional graphene. Enabled by the mixed packing (forming a unique point-to-line-to-plane contact morphology), diverse electrical conduction (forming a well-interconnected three-dimensional electrical conduction network), and varied porosity (forming a well-defined hierarchical porous structure) from these multiple constituents, the optimized quaternary nanocomposite electrode possesses a good combination of facilitated electron transport and enhanced ion diffusion and thus a twofold higher rate capability (capacitance retention: 83.3% vs. 28.9% at 80 A g−1 vs. 0.5 A g−1), twofold higher power density (84.1 kW kg−1 vs. 41.9 kW kg−1), and superior high-rate cycling stability (capacitance retention: 80.2% vs. 65.9% after charge/discharge at 10 A g−1 for 30,000 cycles) than its conventional activated carbon counterpart, while its high volumetric power density (50.5 kW L−1) strengthens its practical usefulness. Our research represents a novel approach to fabricate commercially available materials into practically useful ultrahigh-power supercapacitors at low cost, which may also be employed as a universal strategy to develop high-performance nanocomposites to benefit a wider range of application technologies. [ABSTRACT FROM AUTHOR]