Methylmercury (MeHg) contamination in paddy fields is a significant environmental issue globally since over half of the population of our planet consumes rice. MeHg is a neurotoxin produced by microorganisms in oxygen-limited environments. Microbial effect on MeHg production is a hotspot of research; however, it has been largely ignored how the oxidation–reduction potential (E h) shapes MeHg formation. Here, we elucidated Hg (de)-methylation in a contaminated soil by increasing E h stepwise from −300 to +300 mV using a sophisticated biogeochemical microcosm. At the E h range from −300 to −100 mV, high MeHg concentration and dissolved total Hg (THg) concentration were found due to a high relative abundance of Hg-methylation bacteria (e.g., Desulfitobacterium spp.), acidification, and reductive dissolution of Fe­(oxyhydr)­oxides. At the E h range from 0 to +200 mV, the formation of colloids leads to adsorption of Hg and as a result colloidal Hg increased. MeHg reduction with E h (−300 to +200 mV) increase was mainly attributed to a reduced Hg methylation, as dissolved THg and relative abundance of Desulfitobacterium spp. decreased by 50 and 96%, respectively, at E h of +200 mV as compared to E h of −300 mV. Mercury demethylation might be less important since the relative abundance of demethylation bacteria (Clostridium spp.) also decreased over 93% at E h of +200 mV. These new results are crucial for predicting Hg risks in paddy fields.