Scalable manufacturing of flexible, fiber-shaped energy-storage devices has enabled great technological advances in wearable and portable technology. Replacing inefficient oxides with inexpensive and high-performance oxynitrides with more favorable three-dimensional (3D) structures is critical if the practical applications of these technologies are to be realized. Here, we developed a facile and controllable approach for the synthesis of 3D porous micropillars of molybdenum oxynitride (MON), which exhibit high conductivity, robust stability, and excellent energy-storage properties. Our fiber electrode, containing a 3D hierarchical MON-based anode, yields remarkable linear and areal specific capacitances of 64.8 mF cm-1 and 736.6 mF cm-2, respectively, at a scan rate of 10 mV s-1. Moreover, a wearable asymmetric supercapacitor based on TiN/MON//TiN/MnO2 demonstrates good cycling stability with a linear capacitance of 12.7 mF cm-1 at a scan rate of 10 mV s-1. These remarkable electrochemical properties are mainly attributed to the synergistic effect between the chemical composition of oxynitride and the robust 3D porous structure composed of interconnected nanocrystalline morphology. The presented strategy for the controllable design and synthesis of novel-oxide-derived functional materials offers prospects in developing portable and wearable electronic devices. We also demonstrate that these fiber supercapacitors can be combined with an organic solar cell to construct a self-powered system for broader applications.