Municipal solid waste incineration (MSWI) fly ash contains hazardous metals that may leach into the environment after landfill disposal. This study investigated Akadama soil–mixed geopolymer solidification as a cement-free stabilization and detoxification treatment for MSWI fly ash. Dried fly ash and Akadama soil were mixed at a 1:1 mass ratio, activated with 33 wt% sodium hydroxide solution, molded, and oven-dried to produce geopolymer solidification bodies. Leaching behavior was evaluated for As, Cd, Cr, Pb, Fe, Mn, Sr, and Zn using a standard procedure and a 28-day tank leaching test. Elemental concentrations were determined by ICP-AES. Unconfined compressive strength and resistance to accelerated ultraviolet irradiation were also examined. The treatment successfully produced a solid monolithic matrix and substantially suppressed heavy-metal release. Leached concentrations were below regulatory limits for all investigated elements except Pb. Tank-test concentrations were generally lower than those obtained from powdered samples, demonstrating the importance of monolithic integrity in limiting water contact. The initial compressive strength was 0.800 MPa, below the 0.98 MPa landfill criterion, and decreased to 0.350 MPa after UV exposure. Despite this mechanical deterioration, metal leaching showed no significant increase. Akadama soil–mixed geopolymerization is therefore promising, although strength enhancement and improved Pb immobilization remain necessary.