Manganese (Mn) oxide was prepared by anodizing metallic Mn film that was electrodeposited in n-butylmethylpyrrolidinium bis(trifluoromethylsulfony)imide (BMP–NTf2) ionic liquid. Different anodization courses, namely potentiostatic and cyclic voltammetric methods, led to variations in physical and chemical characteristics of the Mn oxides, and therefore in their pseudocapacitive performance. Evolution of the microstructure, residual weight, and chemical state of the Mn oxides with the charge–discharge cycling number was studied using a scanning electron microscope (SEM), an atomic absorption spectroscope, and an X-ray photoelectron spectroscope, respectively. The analytical results indicate that the electrochemical stability of Mn oxide is mainly determined by its microstructure; the more fibrous (or porous) oxide had a greater durability against cyclic charge–discharge. Moreover, the chemically hydrous state was found to be the most crucial factor that governed the mass specific capacitance of Mn oxide.