Novel multifunctional magnetic gold nanocomposites (MGNCs) were synthesized for synchronous cancer therapy and diagnosis via magnetic resonance imaging (MRI). The MGNCs consist of magnetic kernels (aggregates of ultra-sensitive MnFe2O4 magnetic nanocrystals wrapped in polymer) as effective MR contrast agents and silica–gold nanocomposites as hyperthermal therapeutic agents. A therapeutic antibody, Erbitux (ERB), was conjugated for specific tumor cell targeting both to localize the near-IR laser beam and to image their events through MRI. ERB-conjugated MGNCs selectively recognize the target cancer cell lines. Fluorescence images and MRI analysis show that the MGNCs are effectively taken up by the cells. ERB-conjugated MGNCs have an excellent synchronous therapeutic efficacy as a result of the therapeutic antibody and near-IR laser-induced surface plasmon resonance. Consequently, MGNCs clearly demonstrate selective imaging and treatment of human epithelial cancer simultaneously.