Microbial fuel cells (MFCs) are a potential method for enhanced water and waste treatment, which offer the additional benefit of energy generation. Manganese oxide was prepared by a simple chemical oxidation using potassium permanganate. Carbon-supported manganese oxide nanoparticles were successfully characterised as cathode materials for MFCs. The manganese oxide particles when used in a two-chamber MFC, using inoculum from an anaerobically digested sewage sludge, were found to exhibit similar oxygen reduction performance to that in separate electrochemical tests. MFC tests were conducted in a simple two chamber cell using aqueous air-saturated catholytes separated from the anode chamber by a Nafion membrane. MFC peak power densities were ca. 161 mW m−2 for MnOx/C compared to 193 mW m−2 for a benchmark Pt/C, in neutral solution at room temperature. The catalyst materials demonstrated good stability in the 7.0–10.0 pH range. Theoretical (IR free) peak power densities were 937 mW m−2 for MnOx/C compared with 1037 mW m−2 for Pt/C in the same experimental conditions: showing the MFCs performances can easily be improved by using more favourable conditions (more conductive electrolyte, improved cathode catalyst etc.). Our studies indicated that the use of our low cost MnOx/C catalysts is of potential interest for the future application of MFC systems.