A hierarchical nanostructure composed of NiMn-layered double hydroxide (NiMn-LDH) microcrystals grafted on carbon nanotube (CNT) backbone is constructed by an in situ growth route, which exhibits superior supercapacitive performance. The resulting composite material (NiMn-LDH/CNT) displays a three-dimensional architecture with tunable Ni/Mn ratio, well-defined core-shell configuration, and enlarged surface area. An electrochemical investigation shows that the Ni3Mn1-LDH/CNT electrode is rather active, which delivers a maximum specific capacitance of 2960 F g-1 (at 1.5 A g-1), excellent rate capability (79.5% retention at 30 A g-1), and cyclic stability. Moreover, an all-solid-state asymmetric supercapacitor (SC) with good flexibility is fabricated by using the NiMn-LDH/CNT film and reduced graphene oxide (RGO)/CNT film as the positive and negative electrode, respectively, exhibiting a wide cell voltage of 1.7 V and largely enhanced energy density up to 88.3 Wh kg-1 (based on the total weight of the device). By virtue of the high-capacity of pseudocapacitive hydroxides and desirable conductivity of carbon-based materials, the monolithic design demonstrated in this work provides a promising approach for the development of flexible energy storage systems.