Magnesium doped zinc oxide (Mg_xZn_1−xO) thin films were synthesized on silicon substrate through sol–gel process. Mg_0.15Zn_0.85O thin films were annealed at 500–800 °C and ZnO, Mg_0.1Zn_0.9O, Mg_0.05Zn_0.95O thin films were annealed at 600 °C for 60 min, respectively. The results show that all the samples are of a hexagonal wurtzite structure of ZnO. The surface morphology is strongly dependent on mean grain size and surface fluctuation. Fourier transform infrared spectra reveal that the vibration peak at 420 cm⁻¹ is of the intrinsic lattice absorption of ZnO. The peak at 1083 cm⁻¹ belongs to SiOSi asymmetric stretching vibration. Photoluminescence spectra show that the ultraviolet emission (365–400 nm) and the broad visible emission (469–569 nm) are observed. In particular, Mg_0.05Zn_0.95O thin film annealed at 600 °C exhibits the highest photocatalytic activity, degrading MO by almost 85.8% after 180 min illumination. The photocatalytic activity of the thin film is a synergistic effect defined by grain size, roughness factor, oxygen defects and amorphous MgO.