Ischemic reperfusion (I/R) injury is dominated by excessive reactive oxygen species (ROS)-mediated oxidative damage and uncontrolled inflammation, yet effective strategies for simultaneous diagnosis and treatment remain elusive. Herein, we report a defect-engineered amorphous-like MnCeO nanointerceptor with dual capabilities of magnetic resonance imaging (MRI) -guided stroke diagnosis and ROS-scavenging therapy. The synergistic effect of the amorphous-like structure and Mn-Ce solid solution induces abundant oxygen vacancies and a disordered surface, significantly boosting ROS catalytic removal. Theoretical calculations confirm that Mn doping and oxygen vacancy formation modulate the electronic structure, reduce the adsorption energy of ROS intermediates, and lower catalytic energy barriers, thereby enhancing enzyme-like activity. As a result, MnCeO exhibits an exceptionally high superoxide radical scavenging efficiency (115-fold higher than CeO) and superior MRI contrast (r = 139 mM⁻¹) for precise lesion localization. In vivo, MnCeO efficiently alleviates ROS-mediated oxidative stress and neuroinflammation, promoting substantial recovery from I/R injury. This work offers a powerful defect-engineering strategy for developing next-generation diagnostic and therapeutic nanozymes.