An excited state intramolecular proton transfer (ESIPT) enabled fluorescent sensor phenanthroline diimino phenol (PDP) for Hg²⁺ has been designed and synthesized. PDP acts as a dual sensor and selectively detects only Hg²⁺ in mixed aqueous medium and fluoride in acetonitrile medium over other competing metal ions and anions. The binding of PDP with Hg²⁺ is supported by DFT. The ESIPT phenomenon in PDP is favored in the presence of Hg²⁺, which is rarely reported, along with an intense red fluorescence suppressing other competing metal ions. Among different anionic analytes, only fluoride shows a visually detectable exciting color change from pale yellow to pink with almost similar emission characteristics. PDP also demonstrates its importance in the fluorescent imaging of Hg²⁺ ions in human cancer cells.