In this study, a highly efficient yet low-cost mixed transition metal oxide of Ni and Co as CoO-NiO was synthesized using Azadirachta indica leaf extract as a bioactive chelating agent. The as-synthesized CoO-NiO was further used as a cathode catalyst to improve the performance of a microbial fuel cell (MFC). X-ray diffraction and transmission electron microscopy revealed successful synthesis of CoO-NiO with a cubic symmetry. Cyclic voltammetry analysis showed a clear reduction current peak with a reduction current response of 3.2 mA, whereas the cathode with CoO and NiO synthesized using the same technique also demonstrated a reduction current peak, but with lower current responses of 2.5 mA and 1 mA, respectively. The MFC with the CoO-NiO catalyst could produce a substantially higher power density of 703 mW m-2 at a coulombic efficiency of 22.9%, as compared to the control (57 mW m-2) and the MFC using CoO (616 mW m-2) and NiO (544 mW m-2). In addition, MFCs with CoO-NiO could remove the chemical oxygen demand from synthetic wastewater at an efficiency of 86%, which was the highest efficiency among the tested MFCs. These results illustrated the superiority of the mixed transition metal oxide cathode catalyst to improve the performance of MFCs, and importantly, we utilized a green approach for synthesizing this catalyst, which supports environmental sustainability.