Fibrous morphology and nano-crystalline nature of the anodically deposited manganese oxide were confirmed by a transmission electron microscope (TEM). The oxide electrode exhibited an ideal capacitive behavior as indicated by cyclic voltammetry (CV). In order to explore the energy storage mechanism, variation of electronic and structural aspects of the manganese oxide induced by changing the applied potential was studied in situ in aqueous 2M KCl by Mn K-edge X-ray absorption spectroscopy (XAS). Position shift of the adsorption edge, toward higher energy during oxidation and backward lower energy during reduction, was clearly recognized. The experimental results directly proved for the first time that the pseudo-capacitance of the manganese oxide was attributed to its continuous and reversible faradic redox reaction during the charge–discharge process.