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Expansive soil is a serious problem because it damages civil engineering projects and has an impact on Ethiopia’s road development expansion. In this study, an attempt has been made to assess the effects of corn cob ash (CCA) as a partial replacement of cement for the stabilization of an expansive clay to be used as road subgrade material. Corn cob is a waste agricultural product obtained during the production of corn. After it has been converted to ash, tests are carried out on the pozzolanic property and elemental composition of corn cob ash (CCA). Preliminary tests were performed on the natural soil sample for purposes of classification and identification of some required properties of the sample. Following the required preliminary laboratory analysis, the clay was stabilized with cement and CCA in varying proportions of 2, 4, 6, and 8%, separately. The maximum stabilization effect occurs at 8 and 4% of cement and CCA, respectively. With this percentage, the CBR of the sample increased from 2.62% at 0% to 10.47% and 3.31% at 8 and 4% of cement and CCA, respectively. As a result, 8% of cement was taken as the total amount for different cement and corn cob ash (C: CCA) ratios of 1 : 1, 1 : 2, 1 : 3, and 1 : 4 in the blending stabilization. The optimum blending effect on the strength of stabilized subgrade occurs at a 1 : 2 ratio, containing 2.7% cement and 5.3% CCA. With this ratio, the CBR of the sample increased from 2.62% at 0% to 6.72%. As a result, 5.3% of the cement was substituted with CCA, which had a comparable effect on subgrade strength due to the ideal percentage of cement stabilized. Hence, it can be concluded that CCA can serve as a good complement for the partial replacement of cement in subgrade stabilization.