Mercury (Hg), especially methylmercury (MeHg), is among the most widespread contaminants that are posing potential threat to both humans and wildlife. Coastal seas contribute the majority of human MeHg exposure via coastal fisheries. With the implementation of the Minamata Convention on Mercury since 2017 to reduce anthropogenic Hg emissions, the level of total Hg in the ocean is expected to decrease with time. However, it remains unknown whether and when a similar reduction will be seen in the risk of Hg to marine ecosystems and human health, mainly due to uncertainties in the system efficiency of transforming inorganic Hg discharged into the ocean to methylmercury (MeHg). The terrestrial area surrounding the Eastern China Marginal Seas (ECMS) is one of the Hg emission “hot spots” in the world, resulting in the higher concentrations of Hg in the ECMS in comparison to other marine systems. However, comparable or even lower concentrations of Hg were detected in seafood from ECMS, suggesting a low system bioaccumulation of Hg, which may partially be due to the low system efficiency of producing MeHg (MeHg/THg) in the ECMS. Here, eight cruises in the BS, YS, and ECS were conducted to test this hypothesis. MeHg/THg ratios in ECMS seawater were lower than those in most coastal systems, indicating that the system efficiency of producing MeHg is relatively lower in the ECMS, especially the BS and YS. The low system efficiency of producing MeHg reduces the risk of Hg in ECMS with high Hg discharge intensity. MeHg budget estimation indicates that in situ methylation and demethylation are the major sources and sinks of MeHg in the ECMS. Rapid demethylation and slow methylation of Hg, and low efficiency of transporting inorganic Hg from water column to sediment are major reasons for the low system efficiency of MeHg production in the ECMS. The results of principal component analysis (PCA) and multiple regression analyses performed on global coastal seas and bays showed that system efficiency of producing MeHg was mainly controlled by in situ methylation and transport of inorganic Hg from water to sediment. Sulfate in water and organic matter in sediment were identified to be the key parameters controlling MeHg/THg in global coastal marine systems. This study highlights the necessity of monitoring the system efficiency of producing MeHg and associated processes and controlling parameters to evaluate the efficiency of reducing Hg emissions in China as well as the other countries.