Mortar joint thickness significantly influences the mechanical behavior of masonry structures, yet its quantitative effects remain inadequately characterized. This study investigates the impact of three mortar joint thicknesses (10 mm, 20 mm, and 30 mm) on the compressive and shear strength of clay brick walls through comprehensive experimental testing. Eighteen wall specimens were constructed using locally sourced clay bricks and cement-sand mortar (1:3 ratio) prepared according to EN 1015-3 standards. Mechanical testing was conducted using universal compression testing machines under controlled loading conditions. Results demonstrate that compressive strength decreases from 1.82 N/mm² for 10 mm joints to 1.43 N/mm² for 30 mm joints, representing a 21.4% reduction. Similarly, shear strength decreases by 48% from 1.02 N/mm² (10 mm) to 0.53 N/mm² (30 mm). Statistical analysis (ANOVA, α = 0.05) confirms significant differences between all thickness groups (p < 0.05). Failure mode analysis reveals that thin joints promote brick failure while thick joints fail through mortar joint separation, indicating optimal stress transfer in thinner configurations. The findings establish 10 mm as the optimal joint thickness for maximizing mechanical properties and provide quantitative data for structural design optimization in masonry construction.