AbstractA three‐dimensional porous MnOOH hierarchical nanostructure grown on flexible carbon fiber fabric is successfully synthesized by a facile hydrothermal method. Monoclinic MnOOH has a crystalline structure layered in the [010] direction. The layered structure facilitates intercalation/deintercalation of Li+ into the interlayer space as well as the adsorption/desorption of Li+ onto the surface of each layer during fast charging/discharging process. This porous hierarchical structure provides many electroactive sites and efficient paths for ion transport. When used as flexible and lightweight electrodes for supercapacitors, the porous MnOOH electrode manifests good electrochemical performance with a high capacitance of 160 F g−1 at a current density of 0.2 mA cm−2, a high energy density of 14 Wh kg−1 at a power density of 162 W kg−1, and great cycling stability. Moreover, the application for the low‐cost and highly flexible supercapacitors in powering light‐emitting diodes (LEDs) highlights its enormous commercial potential in next‐generation energy storage systems.