Two-dimensional graphene and its composite nanomaterials offer interesting physical/chemical properties and have been extensively explored in a wide range of fields in recent years. In this work, we synthesize a multi-functional superparamagnetic graphene oxide–iron oxide hybrid nanocomposite (GO–IONP), which is then functionalized by a biocompatible polyethylene glycol (PEG) polymer to acquire high stability in physiological solutions. A chemotherapy drug, doxorubicin (DOX), was loaded onto GO–IONP–PEG, forming a GO–IONP–PEG–DOX complex, which enables magnetically targeted drug delivery. GO–IONP–PEG also exhibits strong optical absorbance from the visible to the near-infrared (NIR) region, and can be utilized for localized photothermal ablation of cancer cells guided by the magnetic field. Moreover, for the first time, in vivo magnetic resonance (MR) imaging of tumor-bearing mice is also demonstrated using GO–IONP–PEG as the T₂ contrast agent. Our work suggests the promise of using multifunctional GO-based nanocomposites for applications in cancer theranostics.