Mercury (Hg) is a toxic pollutant and may cause serious health and environmental threats even at low concentrations. Thus, sensitive, efficient, and accurate techniques for the detection of Hg2+ ions in biological systems are in particular demand. In the current paper, a new, red emitting fluorescence probe (THI) based on electron deficient dicyanovinyl, electron-rich diethylamino, and receptor thiazoline toward Hg2+ has been developed. It has been determined that the recognition behavior of the probe toward Hg2+ is reversible with S2−. The probe not only shows perfect selectivity toward Hg2+ with a low detection limit over a series of metal ions, but it also displays positive solvato-chromism among the tested solvents via modulation of intramolecular energy transfer from the diethylamino to a dicyanovinyl moiety. Furthermore, it has been shown that the probe can be applied as a fluorescent probe for visualizing Hg2+ in living HeLa cells through a confocal laser scanning microscope. Also, the probe THI has not shown any toxic effect in cervical cancer and epithelial cells. Thus, the probe demonstrates high promise for Hg2+ detection in biomarker screening, disease diagnosis, and clinical medicine with low cytotoxicity.