Success in disease therapy depends on precision medicines, where development of formulations with diagnostic and therapeutic functions is quite important. In this study, multifunctional theranostics based on a magnetic graphene oxide (GO) nanohybrid (GIPD) has been developed for magnetic resonance (MR) imaging-guided chemo-photothermal therapy of cancer. The GIPD is endowed with T1/T2 MR imaging capacity via precipitation of small-sized IONP nanoparticles (8.25 ± 2.25 nm) on GO nanosheets through a mild friendly way (60 °C for 1 h, no organic solvent). The obtained nanocomposite is then non-covalently decorated with phosphine oxide polyethylene glycol to improve biosafety. The final nanohybrid effectively loads doxorubicin as the model chemotherapeutic drug and is found to have in vivo T1/T2 MR bimodal imaging functions. Both the in vitro and in vivo results demonstrate that the GO-based nanoplatform displays a good remote photothermal effect, which can damage the dense shell of solid tumor tissue, thereby facilitating the delivery of anticancer drugs into tumor cells. Therefore, this theranostic nanoplatform enables a potent combined chemo-photothermal anticancer efficacy, holding great potential for exploitation of precision cancer therapy.