The contamination of polar regions with mercury that is transported as inorganic mercury from lower latitudes has resulted in the accumulation of methylmercury in the food chain of polar environments, risking the health of humans and wildlife. This problem is likely to be particularly severe in coastal marine environments where active cycling occurs. Little is currently known about how mercury is methylated in polar environments. Relating observations on mercury deposition and transport through polar regions to knowledge of the microbiology of cold environments and considering the principles of mercury transformations as have been elucidated in temperate aquatic environments, we propose that in polar regions (1) variable pathways for mercury methylation may exist, (2) mercury bioavailability to microbial transformations may be enhanced, and (3) microbial niches within sea ice are sites where active microorganisms are localized in proximity to high concentrations of mercury. Thus, microbial transformations, and consequently mercury biogeochemistry, in the Arctic and Antarctic are both unique and common to these processes in lower latitudes, and understanding their dynamics is needed for the management of mercury-contaminated polar environments.