The reduction in strength of stabilized clayey soils due to environmental exposure is a significant challenge in soil improvement. This study investigated the effect of cement content and a 1% (by weight) blend of polyester-polypropylene fibers on the compressive strength and strength retention of kaolinitic soil stabilized with Type II Portland cement. Specimens were prepared with cement contents of 0%, 1.5%, 2.5%, and 4%. After 28 days of curing, they were evaluated under three conditions: a reference dry state, immersion in distilled water, and immersion in a 1% saline solution. Unconfined Compressive Strength (UCS) and Strength Retention Factor (SRF) were determined, and the statistical significance of differences between groups was assessed using one-way analysis of variance (ANOVA) and the Tukey HSD test. Under dry conditions, increasing the cement content from 1.5% to 4% boosted the strength of fiber-free samples by approximately 153%. Furthermore, adding 1% PES–PP fibers to the sample containing 4% cement increased its dry compressive strength by about 15%. Following immersion, the strength of the sample containing 4% cement and 1% fibers reached 1.49 MPa in distilled water and 1.36 MPa in a saline solution; samples immersed in the saline solution exhibited a 16% reduction in strength compared to those in distilled water. Overall, within the range of compositions investigated, the combined use of 4% cement and 1% PES–PP fibers yielded the highest initial strength and the best strength retention following exposure to environmental conditions.