This report develops a coumarin–nicotinic hydrazide-based sensor for detection of CN− ions. The ligand is prepared by simple method and has been characterized by 1H NMR, 13C NMR, FTIR, and mass spectral analyses. The sensing behavior of coumarin–nicotinic hydrazide (CNH) probe was investigated in the presence of different anions using UV–visible and fluorescence methods. The sensor showed a selective naked-eye, colorimetric, and fluorescence detection particularly in the presence of CN− ions over the other anions. The sensing involves a ratiometric red shift in absorption and photoluminescence profile in presence of incremental addition of CN− ions. This provides a multi-detection point at two different wavelengths. This sensor involves a displacement type approach through a nucleophilic substitution-based chemodosimeter sensor proposed based on 1H and 13C NMR along with D2O exchange experiment, and mass and photophysical studies. Interestingly, the sensor affords the lowest limit of detection up to 5.97 × 10–7 M. The practical utility of the sensor is illustrated in molecular logic gate operation and real water analysis.