Mercury (Hg) is one of the most hazardous heavy metals, posing a severe threat to human health. In response to this, we report a new fluorescence probe (HBT-PA) for Hg2+ detection that exhibits strong excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT). HBT-PA exhibits an excellent “turn-on” fluorescence response with high selectivity for Hg2+ ions and negative solvatochromism in many solvent environments. HBT-PA has excellent water solubility, rapid response (10 s), large Stokes shift (203 nm), and high selectivity and sensitivity for Hg2+ over other tested metal ions in water (with 0.01 % DMSO). The probe can detect Hg2+ within the micromolar range (0–50 µM) and has a detection limit of 86 nM. In the presence of CN-, the fluorescence of a solution of [HTB-PA+ Hg2+] was quenched. Thus, the complex could be used to identify trace levels of CN- in water (with 0.01 % DMSO). The probe can be used to visualize Hg2+ ions in food, soil, and water. Moreover, it has been successfully used to develop smartphone applications, cotton swab, and polysulfone capsule kits for Hg2+ detection. Additionally, it has been effectively applied to the visualization of Hg2+ ions in living cells with low cytotoxicity and excellent biocompatibility.