Photocatalytic activity, photovoltaic, and UV detector applications of Zn(1 − x)MgxO (x = 0.02, 0.04, and 0.06) nanoparticles (NPs) and Zn(1 − x)MgxO/graphene nanocomposites were investigated. Graphene oxide (GO) sheets with a few layers were utilized as a graphene source, and the Zn(1 − x)MgxO nanoparticles were synthesized and decorated on graphene oxide sheets by a simple sol-gel method. Transmission electron microscope (TEM) images and X-ray diffraction (XRD) patterns indicated that the Zn(1 − x)MgxO NPs/GO nanocomposites had significantly smaller diameter and crystallite size compared to the Zn(1 − x)MgxO NPs. Several techniques, including XRD, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, and Raman spectroscopy demonstrated that graphene oxide (GO) sheets reduced and changed into reduced graphene oxide (rGO) during the synthesis process. It was observed that the absorption peak of the Zn(1 − x)MgxO/rGO nanocomposites was blue-shifted in comparison with the Zn(1 − x)MgxO NPs. It was discovered that the Zn(1 − x)MgxO/rGO nanocomposites showed an excellent performance to remove methylene blue (MB) dye under natural sunlight illumination compared to the Zn(1 − x)MgxO NPs. Finally photodetector application of the samples under UV source illumination and photovoltaic performance of the samples were also investigated. It was observed a superior solar cell applications for the Zn(1 − x)MgxO/rGO nanocomposites in comparison to the Zn(1 − x)MgxO NPs. Keywords: Zn(1 − x)MgxO/rGO nanocomposites, Graphene, Photocatalyst, Photovoltaic, Ultraviolet detector, Optical properties