Heavy metal contamination can alter both the environmental performance and mechanical stability of engineered soil systems. This study aims to investigate the pH-controlled influence of lead nitrate contamination on the adsorption capacity, consistency limits, and unconfined compressive strength of sand–bentonite and sand–bentonite–zeolite mixtures. Bentonite and zeolite were exposed to lead nitrate solutions at pH 3, 4, and 6, representing extremely high to low contaminant concentrations. Temporal changes in pH and Atterberg limits were evaluated, and lead adsorption capacity was quantified. Subsequently, unconfined compressive strength tests were conducted on sand mixtures containing 20% fines (20% bentonite or 10% bentonite + 10% zeolite) after curing periods up to 28 days. The results show that both adsorbents exhibit mildly alkaline behavior in their natural state but shift toward acidic conditions upon contamination. Lead exposure reduced the plasticity of bentonite due to compression of the diffuse double layer, whereas zeolite exhibited comparatively stable consistency, reflecting its rigid three-dimensional framework. Adsorption efficiency decreased with decreasing pH for both materials; however, bentonite consistently outperformed zeolite, particularly under extreme contamination levels. At low contamination (pH = 6), lead presence enhanced the compressive strength of both sand–adsorbent systems, while severe acidity (pH = 3) resulted in strength degradation, especially in zeolite-containing mixtures. Overall, sand–bentonite mixtures exhibited higher strength gains than sand–bentonite–zeolite mixtures, and all specimens showed pronounced time-dependent strength evolution. The findings highlight the critical role of pH in governing the efficiency of heavy metal adsorption and the mechanical stability of engineered soil systems under extreme contamination conditions.