Reactive magnesia (MgO)-activated ground granulated blast furnace slag (GGBS) is a newly developed binder for soil stabilization/solidification. It can be used as an alternative of conventional hydraulic binders for the stabilization/solidification of heavy metal-contaminated soils. Acid rain exposure has been found to weaken the heavy metal immobilization in solidified/stabilized soils in the long term. However, very limited studies have been conducted to investigate the effect of acid rain on the physical and strength properties of GGBS–MgO-stabilized heavy metal-contaminated soils. In this study, GGBS–MgO-stabilized lead (Pb)-contaminated soils are subjected to the semi-dynamic leaching test using the simulated acid rain (SAR) as leachant, which has pH values of 2.0, 3.0, 4.0, and 5.0. Deionized water (pH 7.0) is also used for comparison. Dry density, soil pH, needle penetration depth, and unconfined compressive strength ( q u ) of the solidified/stabilized soils are measured after the leaching test. The results show that exposure to SAR yields reduction in q u and soil pH, whereas the needle penetration depth increases and dry density changes only marginally. The q u of the solidified/stabilized soils increases noticeably with increased GGBS–MgO content. It is also found that the stabilized Pb-contaminated soil exhibits lower q u than its clean soil counterpart. Furthermore, a quantitative relationship is proposed to correlate the normalized needle penetration resistance with the normalized q u . Finally, a multiple linear regression is performed to statistically reveal the dependence of soil strength on three independent variables, which are SAR pH, Pb concentration and binder content.