Wetlands are critical sites for methylmercury (MeHg) production, a neurotoxin that bioaccumulates in organisms and biomagnifies in aquatic food webs. Net MeHg production in wetland soil depends on the balance between microbially-mediated MeHg production and the degradation of MeHg through both microbial and abiotic pathways. Given that microbial activity is temperature sensitive, climate warming is expected to alter MeHg production in wetland soil. The temperature dependence of both mercury methylation and demethylation processes in boreal wetland soil remains unclear, thus inhibiting our ability to predict future changes. To address this research gap, we conducted a controlled, growth chamber-based experiment in a closed flow-through system incubating wetland soils across ecologically relevant temperatures (5–25 °C). Using additions of enriched mercury isotopes, we simultaneously measured first order rate constants for methylation (km) and demethylation (kd). We found that temperature had a stronger effect on km than kd, with km peaking at 20 °C (~5x higher than the lowest temperature (5 °C)), with changes in km being significantly related to wetland soil respiration, as measured by CO2 and CH4 production. Our study shows that mercury methylation becomes sulfate limited at higher temperatures (25 °C), revealing a substrate availability negative feedback to methylation activity. Thus, climate driven MeHg risks in boreal wetlands will depend on both warming and substrate supply interactions, which can feed back into methylation processes.