We successfully synthesized mesoporous Zn_xCd_1−xS/reduced graphene oxide (Z_xCSG) hybrid materials as photocatalysts using a facile one-pot hydrothermal reaction, in which graphene oxide (GO) was easily reduced (RGO), and simultaneously Zn_xCd_1−xS (Z_xCS) nanoparticles (NPs) with a mesoporous structure were uniformly dispersed on the RGO sheets. By well tuning the band gap from 3.42 to 2.21 eV by changing the molar ratio of Zn/Cd (or Zn content), Z_xCSG with an optimal zinc content has been found to have a significant absorption in the visible light (VL) region. In addition, under VL irradiation (λ > 420 nm), Z_xCSG also showed zinc content-dependent photocatalytic efficiencies for the degradation of methylene blue (MB). Our findings are that, among Z_xCSG, Z_0.4CSG displayed not only a superior photodegradation efficiency of MB (98%), but also good removal efficiency of total organic carbon (TOC) (67%). Furthermore, Z_0.4CSG had a high photocatalytic stability, and could be used repeatedly. The enhanced photocatalytic activity for Z_0.4CSG could be attributed to a synergistic effect between mesoporous Z_xCS NPs and RGO, including the optimal band gap and the moderate conduction band position for Z_xCS (compared to CdS), efficient separation and transfer ability of photogenerated electron/hole pairs in the presence of RGO sheets, and relatively high surface area for both mesoporous Z_xCS NPs and RGO.