<p>Heavy metal contamination has a significant negative impact on ecosystems and living organisms, demanding a need for a simple tool to monitor these pollutants. Mercury is a serious pollutant that enters the environment through both natural processes and industrial activity. Herein, we described a colorimetric mercury sensor based on the principle of the deaggregation of purple gold nanoclusters (AuNCs) to red gold nanoparticles (AuNPs) in the presence of Hg²⁺ and hydrazine hydride. 2-(N-(pyridin-2-ylmethyl)dodecanamido)acetic acid (PyN12G) micelles serve as a template for the formation of AuNCs. Mechanistic investigations using XPS, electrochemical methods, TEM, EDX, DLS, and zeta potential demonstrate that AuNCs catalyze the reduction of Hg²⁺ to Hg⁰ by N₂H₄ on their surfaces, forming a gold amalgam layer that permits deaggregation to form AuNPs. The sensor detects Hg²⁺ over a wide range of pH while also exhibiting excellent specificity and selectivity for Hg²⁺ in the presence of environmental relevant other metal ions. As a proof of concept, we demonstrated the quantitative detection of Hg²⁺ in tap water, pond water, sewage water, sugarcane juice, tomato juice, lime juice, artificial cerebrospinal fluid, artificial saliva, and artificial urine, with a good recovery percentage. The AuNCs-integrated paper was developed to make real-time applications more convenient and simple, and it displayed rapid Hg²⁺ ion detection.</p>