Expansive soils, characterized by pronounced volumetric instability under moisture fluctuations, pose a persistent challenge to the safety, serviceability, and long-term stability of civil infrastructure. In response to these challenges, increasing attention has been directed toward the use of sustainable waste-derived additives as environmentally responsible alternatives for improving problematic soils. This study investigates the mechanical behavior of expansive soil stabilized with waste eggshell powder (ESP) and recycled tire powder (RTP), an elastic polymeric waste material rich in rubber and carbon black derived from discarded tires. A comprehensive experimental program was conducted to evaluate the combined effects of ESP/RTP on soil performance. For this purpose, Atterberg’s limits, compaction, unconfined compressive strength (UCS), Swell potential, and California bearing ratio (CBR) tests were performed on untreated soil and soil modified with up to 25% ESP and 9% RTP. The experimental findings revealed that the incorporation of ESP and RTP significantly reduced the Atterberg limits, optimum moisture content (OMC), and swelling potential of the soil, with the optimum blend of 20% ESP and 6% RTP yielding the most pronounced overall improvement. Such changes signify a considerable reduction in soil plasticity and expansive susceptibility, thereby enhancing the volumetric stability of the treated soil. At the same time, maximum dry density (MDD), UCS, and CBR values increased appreciably, demonstrating improved compaction behavior, greater mechanical resistance, and superior load-bearing performance. Moreover, one-way ANOVA confirmed that the enhancements in UCS and CBR achieved at the optimum mix proportions were statistically significant at the 95% confidence level. Overall, the findings demonstrate that ESP and RTP can serve as a sustainable, cost-effective, and environmentally friendly stabilization solution for improving the engineering performance of expansive soils.