A simplistic and environmentally friendly approach using electrochemically active biofilms (EABs) was developed for the synthesis of an Au–Graphene (Au–G) nanocomposite without the use of surfactants or capping agents. The as-prepared Au–G nanocomposite was characterized by X-ray diffraction, diffuse reflectance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and transmission electron microscopy. In this study, the anchoring of gold nanoparticles (AuNPs) on graphene sheets was achieved using an EAB. The EAB assists in the bio-reduction of Au3+ to Au0, and the AuNPs prevent the aggregation of graphene sheets and keep them apart because of the decrease in attractive forces between the graphene layers. The photocatalytic activities of the Au–G nanocomposite were evaluated by the photocatalytic degradation of methylene blue in an aqueous solution at ambient temperature in the dark and under visible-light irradiation. The photocatalytic activity of the Au–G nanocomposite was enhanced significantly by the loading of AuNPs onto graphene sheets. The photocurrent of the Au–G nanocomposite was measured by linear sweep voltammetry, which exhibited much better performance than pure graphene. The high photocatalytic activity and photocurrent of the Au–G nanocomposite was attributed mainly to the anchoring of AuNPs on the graphene sheets. The synergistic effects of the surface plasmonic resonance of AuNPs and the specific electronics effect of graphene holds great promise for the development of electrochemical devices. Therefore, the Au–G nanocomposite has potential in several fields, such as photocatalysis, photovoltaic, nanoelectronics, ultracapacitors, and sensors because of the enhanced photocatalytic and photoelectrochemical performance.