Electrode materials with high electro-catalytic activity and tailored nanostructures are of great importance for high performance energy storage devices. We develop a novel 3D nanostructured electrode with highly stable, and catalytically-active molybdenum carbide nanoparticles decorated on nitrogen-doped carbon flowers (Mo2C/NCF). The controlled synthesis of Mo2C/NCF provides a uniform distribution of Mo2C nanoparticles of ∼80 nm on carbon microflowers with a self-assembled petal-like structure. The Mo2C/NCF achieves excellent electrochemical performance, with a specific capacitance of 1250 F/g at the current density of 1 A/g in a liquid electrolyte. A device consisting of all-solid-state symmetric supercapacitors (SSC) that used this novel electrode exhibits a high energy density of 54 Wh/kg along with remarkable cycling stability (100% retention after 5000 cycles). We believe that these results provide a new way for carbide materials to be used in high-performance energy storage devices.