Glutathione (GSH) is chosen as a nucleation template to synthesize Pt-based peroxidase nanomimetics ranging from 2.3 to 3.6 nm, of which the surface charge states are greatly associated with the molar ratio of [K₂PtCl₄]/[GSH]. GSH-Pt3.3 consisting of 67% Pt⁰ and 33% Pt²⁺ possesses the highest peroxidase-like activity, while GSH-Pt3.6 comprising 82% Pt⁰ and 18% Pt²⁺ exhibits the most efficient radical scavenging capability. Owing to the Hg²⁺-induced inhibition of peroxidase mimicking activities, a series of Pt-based nanozymes are employed to explore colorimetric assays of Hg²⁺ in aqueous samples with ultrahigh sensitivity. The limit of detection (LOD) can reach 0.25 nM by using GSH-Pt with an average diameter of 3.6 nm. GSH-Pt is a promising candidate for colorimetric assay of Hg²⁺ in both drinking water and biological fluid.