The incorporation of metal ions is found to significantly trigger microstructural changes in clay, altering its mechanical behavior. Four common metal ions, such as monovalent alkali metal ions (Na⁺, K⁺) and divalent heavy metal ions (Pb²⁺, Zn²⁺) were investigated, focusing on how ion valence, type, and concentration influence soil microstructure and mechanical properties. Microstructural tests were conducted through particle size analysis and structural characterization. Macro-mechanical behavior was evaluated using compression and shear tests. Experimental results show that metal ion incorporation reduces particle size and increases clay content. The soil structure is transformed from layered to honeycomb configuration. Small pores are reduced while large pores become predominant. Specific surface area and pore volume are decreased, whereas average pore size is increased. Cation exchange capacity is reduced, but cation adsorption density per unit surface area is enhanced. Monovalent ions are observed to disperse soil aggregates, while divalent ions are identified to induce coagulation. Metal ion contamination is observed to reduce porosity under loading. Soil compressibility is increased with higher ion concentrations. Alkali metal ion-contaminated clays exhibit higher compression coefficients under all load conditions. Heavy metal ion-contaminated clays demonstrate greater compressibility under lower loads. Compared to fieldstate clay, shear strength parameters (internal friction angle and cohesion) of metal ion-contaminated clay are decreased. These reductions are amplified at elevated ion concentrations.