Narrowing the optical band gap of cerium oxide (CeO2) nanostructures is essential for visible light applications. This paper reports a green approach to enhance the visible light photocatalytic activity of pure CeO2 nanostructures (p-CeO2) through defect-induced band gap narrowing using an electrochemically active biofilm (EAB). X-ray diffraction, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and high resolution transmission electron microscopy confirmed the defect-induced band gap narrowing of the CeO2 nanostructure (m-CeO2). The structural, optical, photocatalytic and photoelectrochemical properties also revealed the presence of structural defects caused by the reduction of Ce4+ to Ce3+ as well as an increase in the number of oxygen vacancies. The as-modified CeO2 (m-CeO2) nanostructures exhibited substantially enhanced, visible light-driven photoactivity for the degradation of 4-nitrophenol (4-NP) and methylene blue (MB) compared to the p-CeO2 nanostructure. The enhancement in visible light performance was attributed to defects (Ce3+ and oxygen vacancy), resulting in band gap narrowing and a high separation efficiency of photogenerated electron–hole pairs. Photoelectrochemical investigations also showed the significantly-enhanced separation efficiency of the photogenerated electron-hole charge carriers in the m-CeO2 nanostructure under visible light irradiation. The DC electrical conductivity of m-CeO2 showed higher electrical conductivity than p-CeO2 under ambient conditions. This study provides a new biogenic method for developing narrow band gap semiconductor nanostructures for efficient visible light driven photocatalysis and photoelectrode applications.