Mercury is a contaminant of global concern. It is present widely in global ecosystems and its methylated species is a known developmental neurotoxin to humans. The fate and behaviour of mercury in the environment are largely affected by its speciation, especially among different oxidation states controlled by redox chemistry. Mercury redox chemistry is the key chemical mechanism mediating the transport of the contaminant, yet it has not been well represented in global and regional mercury transport models. Most models rely heavily on computational data to parameterize mercury redox chemistry, and large discrepancies have been reported between the model parameters and experimental results. In this thesis, several critically important mercury redox processes in the atmosphere and marine cryosphere are investigated experimentally in laboratory, mesocosm and field studies. In the atmosphere, in-cloud mercury photoreduction is found to occur at rates that are much slower than those currently used in models, questioning the presumed dominance of the aqueous-phase reduction in the atmosphere. Mercury redox reactions are also studied in an outdoor sea ice mesocosm. At the atmosphere-sea ice interface, saline surfaces of experimental sea ice are shown to support heterogenous photochemical reactions causing the depletion of gaseous elemental mercury in the atmospheric boundary layer; whereas at the sea ice-seawater interface, cryo-photochemical processes could cause the decrease in the concentrations of dissolved gaseous mercury during the formation of sea ice. Overall, the results from this thesis research provide new and important concepts and experimental datasets to the mercury redox mechanism during its geochemical cycle. The results will advance model parametrizations on mercury redox chemistry and improve future projections of mercury cycling in the atmosphere and the Arctic marine cryosphere, which are especially important under a rapidly changing environment. Furthermore, the results also validate the mesocosm approach on studying cryo-photochemical processes in the sea ice environment, which opens up a new platform to study the geochemistry of other contaminants in the marine cryosphere.