Imaging-guided therapy, which bridges treatment and diagnosis, plays an important role in overcoming the limitations of classical cancer therapy. To provide a more exact location of the tumor and to reduce side effects to normal tissues, a multifunctional probe was designed to serve as both an imaging agent and a therapeutic agent. Ternary hybrid nanoparticles comprised of visible red-responsive graphene, the T1-weighted magnetic resonance imaging (MRI) agent Mn3O4 and a mussel-inspired linker polydopamine. The conjugation of graphene to Mn3O4 through polydopamine enhanced the water solubility of Mn3O4, enabling an efficient uptake by cancer cells as well as tumor accumulation when the nanoparticles were intravenously administered into mice. These nanoparticles, when localized at a tumor site, exhibited low cytotoxicity in the dark, while light irradiation of the cancer cells transfected with the nanoparticles resulted in significant phototherapeutic effects, apparently by generating toxic reactive oxygen species. These nanoparticles also allowed excellent T1-weighted MR imaging in a human lung cancer xenograft model and were successfully used for combined visible red-imaging-guided photodynamic therapy and T1-weighted MRI.