This article presents a comprehensive study on the efficacy of xanthan gum (XG) biopolymer as a green construction material in treating problematic expansive subgrade. The soil was treated with various concentrations of XG (0, 0.5, 1.0, 1.5, 2.0, and 5.0%) considering long-term aging period (0, 4, 7, 14, 28, 60 and 365 days). The results showed a reduction in soil plasticity at a higher aging period with the increase in XG content. At 1.5% XG content the unconfined compressive strength, elastic modulus, resilient modulus, unsoaked and soaked CBR values of treated soil were notably increased by 5–10 orders of magnitude and regarded as optimum. Meanwhile, microstructural studies coupled with elemental analysis highlight the bridging of soil particles with stiff XG-hydrogel formation that contributes to strength development. Overall, XG-treatment exhibits an economical and sustainable solution for the treatment of problematic expansive subgrade.