Crude oil contamination poses significant environmental and geotechnical challenges, particularly in oil-producing regions where construction on polluted sites is often unavoidable. Hydrocarbon infiltration alters the engineering behavior of soils, leading to reductions in strength, stiffness, and bearing capacity. In this study, the mechanical stabilization of crude oil-contaminated soil was investigated using two geopolymer slurries prepared from slag and metakaolin. Soil specimens were contaminated with 5%, 10%, and 15% crude oil (by dry weight) and treated with geopolymer contents of 5%, 10%, 15%, and 20%, resulting in 28 experimental mixtures. The effects of treatment were evaluated through Atterberg limits, compaction characteristics (optimum moisture content and maximum dry density) and unconfined compressive strength tests. The results indicated that crude oil contamination significantly deteriorated soil performance, while geopolymer addition markedly improved its engineering properties in a dosage-dependent manner. The 28-day unconfined compressive strength increased by approximately 15–710% compared to untreated contaminated soil. Within the investigated geopolymer contents (5–20%), the greatest strength improvement was observed at 20% geopolymer content. The plasticity index decreased by nearly 42%–90%. The maximum dry density (MDD) of the oil-contaminated soils varied depending on the oil contamination level and geopolymer type, ranging from approximately 5% decrease to 11% increase, while the optimum moisture content (OMC) decreased by approximately 10–30%. Slag-based geopolymer exhibited a faster reaction rate and greater strength enhancement than the metakaolin-based system, attributed to the formation of a denser cementitious gel structure.