Metal-enhanced fluorescence (MEF) is a powerful tool in the design of sensitive chemical sensors by improving brightness and photostability to target-responsive fluorophores. Compounding these advantages with the modest hardware requirements of fluorescent sensing compared to that of centralized elemental analysis instruments, expanding the use of MEF to the detection of low-level inorganic pollutants is a compelling aspiration. Among the latter, monitoring mercury in the environment, where some of its species disseminate through the food chain and, in time, to humans, has elicited a broad research effort towards the development of Hg2+-responsive fluorescent sensors. Herein, a Hg2+-sensitive MEF-enabled probe was conceived by grafting a Hg2+-responsive fluorescein derivative to concentric Ag@SiO2 NPs, where the metallic core enhances fluorescent emission of molecular probes embedded in a surrounding silica shell. Time-resolved fluorescence measurements showed that the fluorophore’s excited state lifetime decreases from 3.9 ns in a solid, coreless silica sphere, to 0.4 ns in the core-shell nanoprobe, granting the dye a better resistance to photobleaching. The Ag-core system showed a sizable improvement in limit of detection at 2 nM (0.4 ppb) compared to 50 nM (10 ppb) in the silica-only colloids and its effectiveness for natural water analysis was demonstrated. Overall, the reported nanoarchitecture hints at the potential of MEF for heavy metal detection by fluorescence detection.