A multifunctional nanocomposite theranostic system is constructed of manganese oxide (MnO) nanoparticles (NPs), as a tumor diagnostic agent, in conjunction with polyacrylic acid (PAA), as a pH-sensitive drug delivery agent, and methotrexate (MTX), as a model of targeting agent and anticancer drug. Physicochemical characteristics of the MnO@PAA/MTX system is studied in detail by several techniques, including X-ray and Auger photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and electrochemical methods. The system performance is studied based on (i) in-vitro MRI measurements to support efficiency of the MnO@PAA NPs as a diagnostic agent, (ii) drug release performance of the MnO@PAA/MTX NPs at pHs of 5.4 and 7.4 through in-vitro method to evaluate application of the NPs as pH-sensitive nanocarriers for MTX, and (iii) impedance spectroscopy measurements to show MnO@PAA/MTX NPs affinity for capturing of cancer cells. The results show that (i) MnO@PAA NPs can be used as a contrast agent in MRI measurements (r ≅ 6.5 mM s), (ii) the MTX, loaded on MnO@PAA NPs, is released faster and more efficient at pH 5.4 than 7.4, and (iii) the GC-MnO@PAA/MTX electrode system captures the 4T1 cells 3.32 times larger than L929 cells.