Magnetic nanoparticles (MNPs) have been extensively used as contrast and hyperthermia agents for magnetic resonance imaging (MRI) and magnetic fluid hyperthermia (MFH) applications. Current superparamagnetic iron oxides, however, exhibit low sensitivity and poor heating efficiency. MNPs should possess precisely tunable magnetic properties and biological functionalities for early diagnosis and efficient therapeutics, which could be achieved by tailoring the MNP size, shape, composition, and surface coating during chemical engineering processes. Recent advances in controllable synthesis that have helped realize promising MNP platforms as high-performance contrast and hyperthermia agents for highly sensitive MRI and efficient MFH applications have been reviewed. All of those dependences should be optimized together in order to reach a comprehensive, conclusive understanding of MNPs and maximize T1 or T2 relaxivity and specific absorption rate (SAR) to chemically engineer an ideal nanoagent.