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[Display omitted] • Benzohydrazide-based chromogenic chemosensors for Hg2+ ion is established. • NMR, FTIR, UV–Vis and HRMS methods were used to confirm the ligand and complex. • PN effective sensing with Hg2+ ions were observed to be 3.08 × 10−7 M detection limit. • Test strip was designed to monitor the absorbance of Hg2+ ions in real time analysis. Metal ions play a crucial role in serving the proper functioning of living beings. Whereas Hg2+ ions are non-essential and toxic even at a minute level for living organisms. Thus, a simple colorimetric approach for Hg2+ ion detection is used. In this work, the synthesis of novel benzohydrazide-based Schiff base ligand (PN) as a sensor and exploring its colorimetric recognition ability. The ligand PN has been designed and synthesized via an economical and simple one-step condensation process. The as-synthesized PN ligand were successfully characterized by various spectroscopic techniques like 1H NMR, 13C NMR, FT-IR, HRMS and ICP-OES. ligand PN manifest outstanding selectivity for Hg2+ ions leaving the rest of the examined metal ions in mixed aqueous medium. A colour change of colourless to crimson yellow is prominent under UV light. The Job plot has shown formation of complex of 1:1 stoichiometric ratio of PN and Hg2+ ions; which is produced by conducting UV–vis titrations using different molar fractions of Hg2+ ions. Moreover, theoretical DFT calculations are validated by experimental data. Furthermore, pre-stained paper strips are furnished and it also showed excellent instantaneous Hg2+ naked eye sensing with a limit of detection as low as 3.08 × 10−7 M. [ABSTRACT FROM AUTHOR]