<p>The comprehensive utilization of industrial solid waste and sustainable soil stabilization are critical for advancing eco-civilization and carbon neutrality. Reactive MgO-GGBS was used as a low-carbon binder of silty clay, and sisal fiber was incorporated into it to enhance its crack resistance. This study investigates the durability and micro-mechanisms of silty clay stabilized with reactive MgO-GGBS and sisal fiber under wetting–drying cycles. The effects of binder ratios (MgO: GGBS = 2:8, 5:5, 8:2), fiber dosages (0‰, 4‰), and curing methods (natural curing followed by carbonation vs carbonation followed by natural curing) were evaluated through physical, mechanical, and microstructural analyses. The results indicate that higher MgO ratios (50%, and 80%) reduced the rates of mass and volume change. This can be attributed to the formation of carbonation products during the curing process, which are relatively insensitive to wetting–drying cycles. Meanwhile, fiber incorporation enhanced toughness, ductility, and residual stress retention, particularly in high MgO ratio samples. Although the first wetting–drying cycle led to a decrease in strength, the samples with high MgO ratios and fibers experienced less strength losses. This was because a three-dimensional network structure was formed, composed of reaction products, soil particles, and sisal fibers. These components are mutually cemented, thereby creating a more stable structure. Microstructural analyses revealed hydrotalcite-like phase expansion in low MgO ratio samples, leading to pore enlargement and structural weakening. Conversely, high MgO ratio samples achieved superior durability through dense carbonation products and fiber synergy. These findings provide novel insights into optimizing alkali-activated GGBS carbonation and natural fiber reinforcement for soft soil stabilization, advancing sustainable geotechnical practices aligned with <i>China’s 14th Five-Year Plan</i> for energy conservation and carbon reduction.</p>