A novel electrochemical route to prepare the Mn oxide with ideal pseudocapacitive performance is successfully proposed in this study. In n-butylmethylpyrrolidinium bis(trifluoromethylsulfony)imide (BMP–NTf2) ionic liquid, a nanostructured and amorphous Mn film could be electrodeposited on a Ni substrate. The metallic Mn films were then anodized in Na2SO4 aqueous solution by the potentiostatic, galvanostatic, and cyclic voltammetric methods and transformed to Mn oxides. It was confirmed that the different anodization courses would cause variations in the material characteristics of the prepared Mn oxides and therefore in their pseudocapacitive performance. The Mn oxide anodized by the cyclic voltammetric method showed the most promising specific capacitance of 402Fg−1; moreover, its capacitance retained ratio after 500 charge–discharge cycles was as high as 94%.