With Co(NO3)2·6H2O (oxidizer and Co source), Ni(NO3)2·6H2O (oxidizer and Ni source) and citric acid (fuel) as starting materials, cobalt–nickel oxides/C/Ni ternary nanocomposites have been synthesized by a scalable, one-step solution combustion process at only 300 °C within 30 min in air. In these composites, the metallic nickel and amorphous carbon (conductive phases) were in situ formed by the reduction of Ni2+ and carbonization of the excess citric acid during combustion, respectively. Experimental results indicated that the fuel:oxidant and Co:Ni molar ratios in precursor solution showed strong influences on the phase composition, morphology and electrochemical performance of products. With the increase of the fuel dosage, the products transformed from well-crystallized cubic NiO/Ni to Ni (nickel-related phases), then to relatively amorphous Ni/NiO and finally NiO. Electrochemical tests indicated that the optimized product showed a high specific capacitance of 446 F g−1 at 1 A g−1 (or 280 F g−1 at 10 A g−1) with a Co:Ni:C6H8O7 molar ratio of 4:5:86/9. Significantly, besides its mild experimental conditions, the method could be used to prepare cobalt–nickel oxides/C/Ni/CNTs quarternary nanocomposites by simply adding acid-treated CNTs into precursor combustion solution. Thanks to the high electrical conduction of CNTs, the specific capacitance could be further improved up to 579 F g−1 at 1 A g−1, or 350 F g−1 at 10 A g−1.