Clayey geomaterials rarely occur in a pure state in nature and are more commonly found as mixed-layer clays such as interstratified illite-smectite. These clays consist of varying proportions of illite and smectite layers, significantly affecting their mechanical properties. Under varying mechanical conditions, the shear behavior of mixed-layer clays exhibits considerable complexity, underscoring the need for in-depth investigations. This study presents a molecular-scale investigation into the behavior of interstratified illite-smectite minerals, simulating a geotechnical shear setup at the molecular level. Multiple molecular models were constructed to explore the effects of water content and illite layer proportions, effectively replicating stages of the illitization process. Results reveal that the water content and illitization play a crucial role in affecting the properties of mixed-layer clays. Models with various illite content show significant difference in molecular cohesion and friction coefficient. Pure illite exhibited the highest shear strength among the studied materials. These findings provide essential insights into the nanoscale mechanical properties of mixed-layer clay minerals, contributing to a deeper understanding of geomaterial stability in critical applications.