Mercury has received widespread attention as a toxic heavy metal contaminant. Organic mercury, especially methyl mercury (MeHg), is more toxic than inorganic mercury and can easily accumulate in the food chain; therefore, the problem of inorganic to organic mercury conversion has attracted much attention in recent years. Most previous research has focused on mercury methylation in marine environments, and research on freshwater environments, especially wetlands, is lacking. The uniqueness and complexity of the wetland environment make it a prominent MeHg production unit, and the bioaccumulation effect is evident. In the present study, the pathways, mechanisms, influencing factors, and related biological effects of organic mercury conversion to inorganic mercury in wetlands are systematically summarized. Sediments, particulate organic carbon, and periphyton organisms are important microenvironments for MeHg generation, and frequent interactions between organisms guarantee the high methylation potential of wetlands. Therefore, strengthening the research on Hg methylation in various wetlands is of great significance for protecting the ecology of wetlands and the health of residents.