Bentonite soil, renowned for its pronounced expansiveness and limited load-bearing capacity, presents notable difficulties in engineering construction due to its vulnerability to volumetric fluctuations. This study investigates the utilization of copper slag, a by-product of copper smelting, as a stabilizer to enhance the geotechnical properties of bentonite soil. Laboratory experiments were conducted to assess the impact of varying proportions of copper slag on soil stabilization, compaction characteristics, and California Bearing Ratio (CBR). The results indicated substantial improvements in soil properties with the addition of copper slag. The study’s objective was to identify the optimal copper slag proportion to enhance soil strength, focusing on maximum dry density (MDD), optimum moisture content (OMC), CBR, and unconfined compressive strength (UCS). The experiments revealed that a mix of 76% bentonite soil and 24% copper slag provided the best stabilization, meeting all subgrade standards as per Indian Standards. This blend significantly improves the soil’s engineering characteristics, making it more suitable for construction purposes. The environmental impact analysis highlighted positive contributions in waste utilization, sustainable construction practices, and dust control, though concerns about leaching of heavy metals and air quality were noted. Predictive modelling confirmed the robustness of using copper slag for soil stabilization, with a high R2 value of 0.98, indicating that 98% of the variability in UCS is explained by the model. This research underscores the practical application of copper slag in enhancing soil properties, contributing to resilient infrastructure development.