Traditional chemotherapeutic drugs have limitations due to their non-targeted ability toward cancer cells. Stimuli-activatable prodrugs are designed to overcome these obstacles. However, the real-time monitoring of stimuli-activatable theranostic prodrugs still poses challenges. Herein, a prodrug (Fe–SS–HCy), consisting of a ferrocene-modified hemicyanine linked via a disulfide bond, is synthesized for anticancer imaging and therapy. Before activation, the toxicity of Fe–SS–HCy is low. The fluorescence of Fe–SS–HCy is quenched by ferrocene due to photoinduced electron transfer. After being taken up by cancer cells, the intracellular GSH activates Fe–SS–HCy, which releases HCy. The fluorescence of HCy is restored and selectively accumulates in the mitochondria, which further produce reactive oxygen species (ROS) to induce cancer cell death. Thus, this “off-on” fluorogenic HCy presents a new strategy for monitoring prodrug activation in real-time and for enhancing therapeutic efficacy with reduced side effects.