In the metropolitan areas of Japan, coastal landfills have become a practical and main solution for the disposal of municipal solid waste incinerator ash (MSWIA) due to limited inland space. Leachate from the landfilled MSWIA contains many dissolved and suspended substances harmful to the surrounding environment, particularly heavy metals, such as lead, zinc, and cadmium. The mobility assessment of these heavy metals is therefore one of the important environmental issues for the coastal landfill. In this paper, modified batch tests were conducted to investigate the effects of pH and E_h changes on the mobility of heavy metals in both MSWIA and marine clay layer. Based on the speciation of heavy metals by using a sequential extraction method in the leachate-MSWIA and leachate-marine clay systems, heavy metal mobility and availability were discussed with the comparison to large-scale model test results presented by (Kamon et al. “Biochemical Effects on the Long-Term Mobility of Heavy Metals in Marine Clay at Coastal Landfill Sites,” J. ASTM Int., Vol. 3, 2006), which provides the long-term change in heavy metal concentrations and forms in the coastal MSWIA landfill site. Under the high pH (≈11) and low E_h (≈−200 mV) condition in the MSWIA-leachate system, Zn, Pb, and Cd were effectively immobilized by the formation of the reducible and oxidizable fractions of Zn and the oxidizable and residual (insoluble) fractions of Pb significantly. However, the potential mobility of Cd was relatively high since the formation of exchangeable compounds was more dominantly contributed to the immobilization. Test results on the speciation of the metals in the marine clay-leachate system accounts well for the evidence that the marine clay acts as the attenuation layer for heavy metals in leachate. These findings strongly support that heavy metal mobility is restricted in the coastal MSWIA landfill due to the formation of stable and insoluble forms under reduced-alkaline condition and the attenuation effect of the marine clay.