Multi-functional core-shell FeO@Au nanoparticles (FeO@Au-DOX-mPEG/PEG-FA NPs) conjugated with doxorubicin (DOX), methoxy poly(ethylene glycol) (mPEG), and folic acid-linked poly(ethylene glycol) (PEG-FA) were synthesized for cancer theranostic applications. In the developed NPs, the DOX was chemically conjugated at the surface of core-shell FeO@Au NPs using L-cysteine methyl ester (LCME) as a linker by acid-sensitive hydrazone bond. The formation of FeO@Au-DOX-mPEG/PEG-FA NPs was confirmed by H-NMR analysis. The TEM image and DLS studies showed that the mean diameter of the prepared NPs was about 18 and 38 nm, respectively. Due to the existence of superparamagnetic FeO, the FeO@Au-DOX-mPEG/PEG-FA NPs presented the saturation magnetization (Ms) value of 23 emu/g. The developed NPs displayed the maximum amount of drug release in the acidic medium than that in the mild alkaline medium because of the presence of acid-sensitive hydrazone bond. Due to the presence of FA, the FeO@Au-DOX-mPEG/PEG-FA NPs displayed the increased cellular uptake through a folate-receptor-mediated endocytosis, which results in the improved cytotoxic effect on the HeLa cells. Under the laser irradiation, the cytotoxicity of FeO@Au-DOX-mPEG/PEG-FA NPs was found to be improved due to the photothermal effect of Au shell existing in the NPs. These results reveal that the FeO@Au-DOX-mPEG/PEG-FA NPs could be a promising tumour-targeted drug delivery system with the capabilities of combined MR/CT imaging, photothermal, and chemotherapy of tumours.