Environmental monitoring of mercury ions (Hg²⁺) and hydrazine (N₂H₄) is crucial due to their extreme toxicity and persistence. Herein, we report a newly designed dual-reactive fluorescent probe capable of selective and sensitive detection of both Hg²⁺ and N₂H₄ through distinct optical responses. The sensing mechanism involves inhibition and activation of intramolecular charge transfer (ICT), excited-state intramolecular proton transfer (ESIPT), and photoinduced electron transfer (PET) processes, leading to fluorescence “turn-on” at 610 nm for Hg²⁺ and 478 nm for N₂H₄. Exposure to Hg²⁺ causes desulfurization and cleavage of the phenyl carbonothioate (PCS) group, restoring conjugation and yielding orange emission, while N₂H₄ triggers nucleophilic addition–cyclization to a pyrazole derivative with cyan emission. Spectroscopic and kinetic analyses confirmed rapid, linear responses under physiological conditions (DMSO/PBS, pH 7.4). ¹H NMR TOF-MS, and FT-IR data verified structural transformations, supported by DFT calculations. The probe showed high selectivity, strong anti-interference capacity, and effective sensing at neutral pH. The probe exhibited excellent performance in environmental matrices, allowing visual detection of Hg²⁺ and N₂H₄ in soil, plant tissues, and real water samples under UV light. The probe-coated TLC sheets, thin films, and patterned papers enabled on-site and portable sensing, while cytotoxicity assays confirmed its low toxicity and biocompatibility. Overall, the probe provides a robust, dual-emissive fluorescent system for the rapid and visual detection of mercury and hydrazine, offering a practical tool for environmental monitoring and pollution diagnostics.