Mercury is one of the most hazardous heavy metals and is capable of biomagnification, thereby posing severe risks to ecosystems and human health. Therefore, selective, sensitive, and rapid detection of Hg2+ in a wide range of samples is essential. Herein, we report the synthesis of a new 2-(benzo[d]thiazol-2-yl) phenol-based fluorescent probe (PyS) and its potential as a fluorescent probe for detecting Hg2+ ions in various real samples such as rice, garlic, shrimp, and root samples. When interacting with Hg2⁺, the non-fluorescent probe solution emitted strong red fluorescence at 638 nm in a solution of DMSO/H2O (1/9, v/v). The other ions showed no significant interference during Hg2+ detection. In addition, PyS displayed a rapid response time among the reaction-based systems (3–4 min), low detection limit (72.5 nM), linear response trend in the range of 0–22.00 μM, and large Stokes shift (243 nm). Additionally, PyS can serve as a paper, cotton swab, and polysulfone capsule kit for qualitative detection of Hg2⁺, enabling on-site quantitative detection of Hg2⁺ in beach soil and water samples using a smartphone app. It also can effectively detect Hg2⁺ in living cells in a concentration-dependent manner, thereby highlighting its potential for bioimaging in living organisms.