Metal-organic frameworks (MOFs) are versatile materials used in adsorption and separation. Post-synthetic modification, especially solvent-assisted linker exchange (SALE), enables precise ligand functionalization while maintaining framework integrity. However, incorporating linkers with Lewis basic sites presents challenges due to their strong coordination tendencies, which can disrupt framework stability. In this work, a ligand additive-stabilized SALE is introduced to incorporate 2-mercaptoimidazole into zeolitic imidazolate framework-8 (ZIF-8), preserving its crystallinity and porosity while enabling controlled functionalization. A systematic study is conducted to correlate SALE parameters-including exchange ligand concentration, reaction time, and solvent environment-with morphological changes and ligand incorporation efficiency. Quantitative image analysis of MOF particle curvature revealed the relationship between ligand exchange dynamics and framework stability, providing insights into SALE-induced structural transformations. The sulfur-functionalized ZIF-8 exhibits a high Hg adsorption capacity of 1504 mg g and excellent recyclability. It showed Hg removal efficiency (>95%) across a wide pH range and retained 95% of its initial capacity after cycles. In a multi-ion system containing additional metal ions, the material demonstrated ≈100% removal efficiency for Hg, confirming its practical applicability in complex environments. This ligand additive-stabilized SALE provides a reliable approach for introducing sulfur functionalities into MOFs, opening opportunities for enhanced environmental remediation.