Display omitted •Leaching behavior and environmental risk of Mn+ in FA exposed to MLL were studied.•Early leaching of MLL increased the OPTI level of Mn+ in liquid-phase.•The ability of FA3 in reducing STI level of Mn+ in solid-phase was better than FA2.•Non-adsorptive DOM-Mn+ was the main complex form in the early leaching stage.•HJ/T300-2007 was more effective than MLL in enhancing FA exposure scenario. Solidification/stabilization pretreatment + landfill disposal in municipal solid waste (MSW) landfill sites is a widely accepted MSW incineration (MSWI) fly ash (FA) management strategy in China. However, in reality, the stability of FA disposed in MSW landfill sites may be affected by the organic landfill leachate environment. The purpose of this study was to explore the mobility and environmental risks of six toxic metals (Mn+, Pb/Zn/Cu/Cd/Cr/Ni), from raw and solidified/stabilized FA, by simulating a leaching environment with mature landfill leachate (MLL). The leaching of Mn+ mainly occurred in the early leaching stage, and their leaching behavior was controlled by the diffusion of surface Mn+ in the FA matrix. The destructive effect of dissolved organic matter (DOM) on the local precipitation-dissolution equilibrium of FA-leachate interface, the formation of non-adsorptive DOM-Mn+ complex (easy to migrate), and the competitive effect of DOM on the binding sites of Mn+ on the surface of the FA matrix may play an important role in increasing the leaching level of most Mn+. By contrast, the potential of solidified FA in reducing the environmental risk level of leached Mn+ was better than that of stabilized FA. However, the immobilization capability of solidification/stabilization pretreatment on various types of Mn+ in FA should be judged according to their practical disposal environment. Compared to MLL leaching tests, Acetic Acid Buffer Solution Method (HJ/T300-2007) can effectively strengthen the exposure environment and provide a reliable reference level of environmental risk for MSWI FA disposed in MSW landfill sites.