Display omitted •The prepared Fe3O4@ZnO nanocomposite possesses excellent peroxidase mimic activity.•Mechanism is based on the catalytic activity of nanocomposite and antiradical ability of biomolecule.•This colorimetric sensor is highly selective and sensitive for the detection of Hg2+.•The proposed sensing strategy is simple, onsite portability and visual detection by naked eye.•The colorimetric sensor was successfully applied in detection of Hg2+ in real water samples. A novel significant colorimetric sensor has been developed for the selective detection of Hg(II) ions based on Fe3O4@ZnO nanocomposite as peroxidase mimics. We have successfully synthesized nanocomposite by solvothermal method and it is characterized by analytical technique using scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and vibrating sample magnetometry. This nanocomposite exhibits a peroxidase mimetic activity that could efficiently catalyze the oxidation of tetramethylbenzidine (TMB) in the presence of H2O2 to form blue colored cation radical. In addition, the biomolecule of Cysteine (Cys) easily hinder the cation radical formation and turns back TMB molecules into colourless. Whereas the Hg(II) ion, having strong affinity to thiol group easily binds with Cys. As a result of it, the blue colour of the solution is recovered in presence of Fe3O4@ZnO nanocomposite. Based on this mechanism, we have developed a simple and rapid colorimetric detection of Hg2+ ions, which is found to be 23 nM. Furthermore, the colorimetric sensor has been applied to detect Hg2+in real water samples and find convenient potential applications in the biological and environmental field.