A 3D graphene sheet–carbon nanotube (GS–CNT) structure with a good wetting property, high porosity, and large surface area is homogeneously deposited with active amorphous manganese oxide (a-MnO x ) by potentiodynamic deposition. The flowery a-MnO x nanostructure with ultra-slender petals (ca. 5–8 nm) on the framework of hierarchically porous GS–CNT matrix not only enables nearly full utilization of a-MnO x but also retains sufficient conductivity and porosity for the high-rate charge–discharge application. The use of a-MnO x on the 3D GS–CNT material produces a specific capacitance of MnO x of 1200 F g−1 which is much-higher than that of a pure a-MnO x electrode (C S,Mn = 233 F g−1). The specific energy and specific power of a-MnO x /GS–CNT are respectively as high as 46.2 Wh kg−1 and 33.2 kW kg−1, revealing that our work conceptually provides a way to produce porous structures composed of graphene, carbon nanotubes, and various electroactive materials for high-performance energy storage devices.