The development of nanotechnology for hazardous heavy metal detection with nanoparticles (NPs) created an interest for the preparation of MnS/Ag nanocomposite. Here, MnS/Ag-polyvinylpyrrolidone (PVP) nanocomposite was developed for the detection of mercury. The prepared composite was analyzed using particle size analyzer, X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), Ultraviolet-visible (UV–vis) spectroscopy, zetasizer, high resolution transmission electron microscope (HRTEM) and Fourier-transform infrared spectroscopy (FTIR). The λmax of MnS/Ag-PVP nanocomposite was observed at 404 nm. The particle size was determined to be 21 ± 1.7 nm and the surface charge was −31.19 ± 3 mV. The brownish yellow colour of the nanocomposite changed into colourless when Hg2+ was added. The different metal ions present with the analyte did not show any interference on detection of Hg2+. The MnS/Ag-PVP nanocomposite incorporated paper and gel exhibited visual detection of Hg2+ from aqueous sample. There was an excellent linearity (y = −0.0015x + 0.8744) found in plot of 20 to 100 nM Hg2+ concentrations versus absorbance at 404 nm and the LOD was calculated to be 16 nM. The probe was applied to quantify Hg2+ from spiked environmental sample and the results were further confirmed with atomic absorption spectrophotometric analysis. Hence, the investigation suggests that the present probe could efficiently detect and quantify Hg2+ at nano molar level. In addition, the study suggests that MnS/Ag-PVP nanocomposite exhibit multifunctional property including efficient photocatalytic and antimicrobial activity. The antibacterial activity was evaluated against both gram positive and gram negative bacteria.