Thawing permafrost in northern regions threatens to mobilize large mercury and dissolved organic carbon (DOC) stores from permafrost soils. Once mobilized, this inorganic mercury can be transformed (methylated) by microbes into methylmercury, a neurotoxin that bioaccumulates up aquatic food webs. Permafrost thaw may increase methylation in peatland complexes as elevated and dry permafrost peat plateaus thaw into waterlogged nutrient-poor bogs or nutrient-rich fens. Bogs and fens can be favorable environments for microbes responsible for methylation. Transport of mercury and methylmercury is often facilitated by binding to DOC and as peatlands become increasingly hydrologically connected with continued thaw, increased export of DOC, mercury, and methylmercury to downstream inland waters may occur. In this study, we completed three sampling campaigns of 95 streams within the Northwest Territories and Northern Alberta (summer 2021 and 2022) to investigate the role of catchment characteristics, including permafrost extent, on downstream methylmercury concentrations. We found that streams in more southern permafrost regions had higher DOC, mercury, and methylmercury concentrations than streams farther North. We also investigated the influence of catchment characteristics such as land cover and size, and the influence of water chemistry and the quality of DOC on stream methylmercury concentrations. Understanding factors that influence mercury export in Northern rivers is required to forecast the risk of permafrost thaw on the health of aquatic systems and Northern peoples.