Water-soluble and dual-emitting bovine serum albumin-templated bimetallic platinum–gold fluorescent nanoclusters (BSA–Pt–Au NCs) have been synthesized for the first time by a simple and rapid one-pot synthesis strategy at a vital molar ratio of Pt/Au precursors under basic conditions. The BSA–Pt–Au NCs show unique and well-resolved dual-emission bands at 405 nm and 640 nm corresponding to the emission of the BSA–metal complex and Au NCs under the excitation at 340 nm, respectively. Interestingly, with the addition of mercury(II) (Hg²⁺) ions, the emission intensity at 405 nm (F₄₀₅) has a negligible influence, while the emission intensity at 640 nm (F₆₄₀) decreases dramatically, which is due to the specific and strong d¹⁰–d¹⁰ interactions between Hg²⁺ ions and Au atoms/ions on the surface of the BSA–Pt–Au NCs. Meanwhile, cysteine (Cys) could effectively block Hg²⁺ ions to quench the fluorescence of BSA–Pt–Au NCs, which is based on the strong interaction between Hg²⁺ ions and the thiol functional group. These allowed the analysis of Hg²⁺ ions and Cys with the ratio of two emission intensities (F₄₀₅/F₆₄₀) in ultra-sensitivity and selectivity with high accuracy. The limit of detection (LOD) was 0.3 nM for Hg²⁺ ions and 0.04 μM for Cys (S/N = 3), respectively. This work indicates that the fluorescent sensor possesses potential applications for biological detection.