Novel manganese graft ionomer complexes (MaGICs) that contain Mn ions complexed with a polyaminobisphosphonate-g-poly(ethylene oxide) (PEO) copolymer were developed for use as T1-weighted contrast agents for MRI. The complexes exhibited good colloidal stability without release of free manganese and did not result in any in vitro toxicity against mouse hepatocytes. T1 relaxivities of the MaGICs at physiological pH were 2-10 times higher than that of a commercial manganese-based positive contrast agent. Anticancer drugs including doxorubicin, cisplatin and carboplatin were successfully encapsulated into the MaGICs with high efficiency. Drug release behavior was sustained and depended on pH (faster in acidic environments), drug structures and drug concentration (faster with high concentration). The anticancer drug-loaded manganese nanocarriers exhibited excellent anticancer activity against MCF-7 breast cancer cells together with high relaxivity. Thus, these drug-loaded MaGICs could potentially be utilized for simultaneous diagnosis and treatment of cancer.