For ratiometric, fluorogenic and “naked eye” detection of the environmentally and biologically important ions Hg²⁺ and OCl⁻ in one platform, a 2-(2′-hydroxyphenyl)-benzothiazole (HBT) inserted thiolactam containing a new rhodol system, spirothiolactamized benzothiazole substituted N,N-diethylrhodol (STBR), is designed and synthesized. We can detect Hg²⁺ and OCl⁻ in solutions by a simple, easy-to-synthesize practical ratiometric chemodosimeter with two different modes of reactions with a low detection limit. The observed color in the presence of Hg²⁺ or OCl⁻ may arise from the reaction of Hg²⁺ or OCl⁻ with STBR – the thiolactam moiety is broken and it helps to achieve maximum π–π* conjugation which results in a brilliant red fluorescence. From ¹H-NMR and ESI-MS data, it is clear that the modes of the reaction for Hg²⁺ and OCl⁻ are different. Test strips are also prepared to observe color changes. The sensing phenomena are fruitfully applied for live-cell imaging in Saccharomyces cerevisiae cells (from a short to long time range, 0 min to 120 min). The single crystal X-ray structure of the synthesised ESIPT rhodol moiety is also reported.