Here, we describe the visual colorimetric detection of Hg²⁺ based on a flexible fluorescent electrospun nanofibrous membrane (NFM). It is an efficient approach, in which we have effectively integrated fluorescent gold nanoclusters (AuNC) into electrospun polyvinyl alcohol nanofibers. Interestingly, the resulting composite nanofibers (AuNC*NFM) are shown to retain the fluorescence properties of AuNC and exhibit red fluorescence under UV light, being cogent criteria for the production of a visual colorimetric sensor. Furthermore, capabilities with regard to the stability of the AuNC*NFM have been under observation for a period of six months, with conditions matching those of typical atmosphere, and the resulting outcome has thrown light on their long-term storability and usability. It is clear, from the fact that the nanofibrous membrane preserves the fluorescence ability up to a temperature of 100 °C, that temperature does not have an effect on the sensing performance in real-time application. The water-insoluble AuNC*NFM have been successfully tailored by cross-linking with glutaraldehyde vapor. Further, the contact mode approach has been taken into consideration for the visual fluorescent response to Hg²⁺, and the observed change of color indicates the utility of the composite nanofibers for onsite detection of Hg²⁺ with a detection limit of 1 ppb. The selectivity of the AuNC*NFM hybrid system has been analyzed by its response to other common toxic metal interferences (Pb²⁺, Mn²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Cd²⁺) in water. Several unique features of the hybrid system have been determined, including high stability, self-standing ability, naked-eye detection, selectivity, reproducibility and easy handling – setting a new trend in membrane-based sensor systems.