ZnO-graphene nanocomposites have potential applications in sensors, photocatalysis, solar cells, and energy storage. Currently, graphene-ZnO nanocomposites are prepared via multistep solution-based methods that suffer from low productivity and generation of large quantities of liquid waste. In this article, we report a new solventless process for the synthesis of ZnO-graphene nanocomposites contain 9 nm ZnO nanoparticles uniformly dispersed on the surface of thermally reduced graphene (TRG) via short-time ball milling of hydrozincite and graphite oxide (GO) followed by thermal annealing. Characterization of the structure and morphology of the graphene-ZnO nanocomposite as examined by XRD, FTIR, XPS, SEM, TEM, BET, and TGA revealed the ball milled nanocomposite has higher surface area and smaller ZnO particle size compared to the nanocomposite prepared by the solution-based hydrothermal method. Moreover, the photocatalytic activity of the ball-milling composite for degradation of methylene blue (MB) dye under visible light illumination are superior not only to TRG, ZnO, and hydrothermal ZnO-graphene nanocomposite but also to other reported ZnO and ZnO-graphene nanocomposite. Our results clearly demonstrate this mechanothermal process provides a facile, green, and scalable route for development of high-performance metal oxide-graphene nanocomposites.