In this study, the enhancement of photocatalytic activity of a graphene oxide/ZnO (GO/ZnO) nanohybrid was achieved via introducing Mg as a co-dopant element by the co-precipitation method. The Mg-doped ZnO nanoparticles were adsorbed on GO sheets. Compared with the catalyses with GO/ZnO nanohybrids, GO/Zn 1−x Mg x O nanohybrids exhibited better photocatalytic activity, in which the decomposition rate of methyl orange (MO) reached 100% within 60 min. The enhancement is attributed to Mg-doping and efficient charge transfer of the photogenerated electrons in the conduction band of ZnO to graphene.