Expansive soils present significant challenges due to montmorillonite, which causes swelling and shrinkage upon contact with water. These soils requires stabilization due to their high plasticity, low shear strength, and high compressibility. Various stabilizers are employed to enhance the soil characteristics. This study investigates the use of fly ash (FA) and mango kernel ash (MKA) as stabilizers. Initially, the basic properties of the soil are determined. Subsequently, MKA is added to the soil in percentages of 10, 20, 30 and 50. Atterberg limits and unconfined compressive strength (UCS) tests are conducted on soil samples mixed with MKA. The soil combined with 20% MKA exhibits the maximum UCS after 14 days of curing, thrice that of pure soil. FA is mixed with MKA in different ratios. MKA is added to the soil at a constant percentage of 10%, while FA is added in percentages of 0, 10, 20, and 30% by soil weight. Similarly, MKA is kept constant at 20 and 30%, while FA is added in the same percentage range. This results in fourteen different combinations. Among these, the soil mixed with 20% MKA and 20% FA demonstrates an increase in UCS compared to pure soil, four times greater than untreated soil. This treatment induces the formation of calcite and calcium silicate gel, which XRD and SEM validated. This study demonstrates the effectiveness of using waste ashes to enhance the strength of expansive soil.