Peatlands are globally important ecosystems where inorganic mercury is converted to bioaccumulating and highly toxic methylmercury. Although biological mercury methylation has been known for decades, we still have an incomplete understanding of organisms and processes driving methylation in different environments. Numerous studies have demonstrated that wetlands are “hotspot” of Hg methyltion. However, wetlands are diverse with respect to hydrology and biogeochemistry and thus the prevailing environmental conditions that control methylation and demethylation processes are highly variable. We hypothesize that the changes in the trophic status of peatlands across the succession will also be expressed in the composition of their microbial communities and thus in their potential to form MeHg. Here we used experimental laboratory incubations of peat samples from a mire chronosequence with fundamentally different ecohydrological conditions to identify master controls of mercury methylation and demethylation. The results showed that the potential mercury methylation rates decreased with the age of the peatlands, being up to 53 times higher in the youngest peatland compared to the oldest. Methylation in young mires was driven by sulfate reduction, while methanogenic and syntrophic metabolism became more important in older systems. We also show that while biotic methylmercury degradation was important in young mires, abiotic degradation was prominent in older systems. Our work demonstrates a new perspective on Hg(II) methylation across the succession of boreal peatlands which can be used to conceptualize our understanding of the dominating ecohydrological and biogeochemical controls on mercury methylation and the MeHg supply to downstream surface waters.