Alkali-activated slag (AAS) systems offer a sustainable approach for valorizing industrial solid wastes, thereby lessening the environmental impact of both cement production and landfilling. Although granite (GN) and dolomite (DM) wastes are abundant by-products, their use in AAS materials for performance enhancement has not been systematically investigated. This study therefore examined the utilization of GN and DM wastes to improve mechanical strength development in AAS systems. The compressive strength and microstructural evolution were systematically analyzed after replacing 50 wt.% of ground granulated blast furnace slag with GN and DM at varying ratios. Results demonstrate that a GN-to-DM ratio of 2:3 maximizes compressive strength, achieving a 28 d value of 37.21 MPa. The underlying mechanism involves a two-stage process: DM contributes to early strength gain by rapidly releasing calcium (Ca²⁺) and magnesium (Mg²⁺) ions, thereby accelerating the formation of dense reaction products. Subsequently, the aluminosilicate network of GN optimizes the gel composition, promoting mid- and long-term stability. At a GN substitution level of 20 wt.%, the synergy between GN’s slowly reactive aluminosilicate properties and DM’s ionic contribution achieves balanced reactivity and enhanced durability. In contrast, systems relying solely on GN substitution form a poorly developed, Ca-deficient calcium-aluminosilicate hydrate (C-A-S-H) gel because of insufficient calcium availability, resulting in reduced mechanical performance. These findings provide mechanistic insights into the eco-friendly recycling of GN–DM wastes for producing high-performance AAS materials.