Mixed transition metal oxides have attracted great attention due to their improved properties over simple oxides in energy storage applications. Herein, we report facile hydrothermal synthesis of novel porous NiCoMn ternary metal oxide flakes with reduced graphene oxide (NCMO_rGO) and their application in high performance aqueous-based energy storage devices. The effect of reaction time on composition and morphology was studied by X-ray diffraction, Raman spectroscopy, thermal gravimetric analysis, transmission electron microscopy, and N2 ad/de-sorption measurements. Accordingly, a high specific surface area of 200 m2 g−1 was achieved for the optimized sample. The sample was electrochemically investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy. Porous NCMO_rGO hybrid exhibited a high capacity of 115 mAh·g−1 at 1 A g−1. Subsequent integration of the hybrid material as a positive electrode with rGO negative electrodes resulted in asymmetric aqueous devices having a specific energy as high as 27 Wh·kg−1. Moreover, this NCMO_rGO//rGO hybrid device showed an excellent cycling stability of 96% over 2000 cycles. This work not only reports for the first time the rational design and fabrication of porous NCMO_rGO nanoflakes as high performance electrodes for energy storage but also paves the path towards facile synthesis of mixed metal oxide nanocomposites for other applications.