The authors describe MnO nanoparticles (NPs) with unique excitation-dependent fluorescence across the entire visible spectrum. These NPs are shown to be efficient optical nanoprobe for multicolor cellular imaging. Synthesis of the NPs is accomplished by a thermal decomposition method. The MnO NPs exhibit a high r1 relaxivity of 4.68 mM−1 s−1 and therefore give an enhanced contrast effect in magnetic resonance (MR) studies of brain glioma. The cytotoxicity assay, hemolysis analysis, and hematoxylin and eosin (H&E) staining tests verify that the MnO NPs are biocompatible. In the authors’ perception, the simultaneous attributes of multicolor fluorescence and excellent MR functionality make this material a promising dual-modal nanoprobe for use in bio-imaging.