Microbial fuel cells (MFCs) provide an opportunity to harvest electrical energy from biodegradable organic wastes. In air-cathode MFCs, cathode plays an important role at which oxygen reduction reaction (ORR) occurs. Herein, MnO2 nanorods are demonstrated as a potential air-cathode catalyst in a single-chambered MFC (sMFC). Synthesis of two different MnO2 morphologies (nanoflakes and nanorods) is demonstrated at room temperature using a simple precipitation method. In order to improve the performance of MnO2 as ORR electrocatalyst, graphene oxide (GO) is used as conductive matrix instead of carbon black. Furthermore, MnO2/GO is hydro thermally treated to obtain MnO2/reduced GO (h-MnO2/rGO) nanocomposite. The sMFC comprising air-cathode with an optimum loading of h-MnO2/rGO delivers 2.7 times higher volumetric power density than that of sMFC without a catalyst and comparable to the state-of-art Pt/C. This is ascribed to superior 4-electron ORR pathway by h-MnO2/rGO and improved charge transport behavior of rGO support as confirmed through detailed electrochemical investigation. This work demonstrates a facile room temperature synthesis process of MnO2 nanoflakes and nanorods, and their role as air-cathode electrocatalyst for competent power generation in sMFC.