A simple soft bi-templating process was used for the synthesis of mesoporous manganese oxide nanostructures using KMnO₄ as a precursor and polyethylene glycol and cetyltrimethylammonium bromide as templates in the presence of benzaldehyde as an organic additive in alkaline media, followed by calcination at 400 °C. X-ray diffraction and Raman spectroscopic analysis of the calcined products confirmed the existence of stoichiometric (MnO₂ and Mn₅O₈) and non-stoichiometric mixed phases (MnO₂ + Mn₅O₈) of Mn oxides obtained by tuning the concentration of the additive and the synthesis time. The surface properties of the prepared Mn oxides were determined by X-ray photoelectron spectroscopy. The mesoporosity of the samples was confirmed by N₂ adsorption–desorption. Different synthetic conditions resulted in the formation of different morphologies of the Mn oxides (α-MnO₂, Mn₅O₈, and α-MnO₂ + Mn₅O₈), such as nanoparticles, nanorods, and nanowires. The synthesized mesoporous Mn oxide nanostructures were used for the catalytic oxidation of the harmful air pollutant carbon monoxide. The Mn₅O₈ nanoparticles with the highest Brunauer–Emmett–Teller surface area and the non-stoichiometric manganese oxide (α-MnO₂ + Mn₅O₈) nanorods with a higher Mn³⁺ concentration had the best catalytic efficiency.