Arctic rivers deliver~40 t yr⁻¹ of mercury (Hg) to the Arctic Ocean, ~6%of which is from the Mackenzie River Basin(MRB), a region warming at ~3 times the mean hemispheric rate. How this will affect Hg transfer to ecosystems of theBeaufort Sea is a worrying issue. To help address this question, we analyzed >500 measurements ofHg and other waterproperties from 22 rivers collected in 2012–2018 by communities of the MRB. This new dataset provides a more comprehensiveview of riverine Hg variations across the basin than was previously available. We find that rivers issuedfrom mountains in the western MRB contribute the largest share of Hg in the Mackenzie River, 60–95 % of it beingcarried as fine suspended solids and probably sourced from riverbank erosion and thaw slumps. In contrast, lowlandrivers of the central and eastern MRB contribute larger shares of dissolved Hg (up to 78 %), likely from recent atmosphericdeposition through precipitation. Using load modelling constrained by the new water quality dataset, we estimatethat the three largest western tributaries (Liard, Peel and Arctic Red rivers) of the Mackenzie contribute 60 % ofthe annual MRB THg export and DHg export to the Beaufort Sea during freshet, as well as 51 % of DHg export, whilesupplying 60% of freshet discharge. Load modelling also reveals a sustained decline in DHg loads of ~13 kg yr⁻¹ between2001 and 2016 in the lower Mackenzie River, which likely reflect a decreasing trend in atmospheric Hg depositionover most of northwestern Canada during this period. This study highlights the value of community-based waterquality monitoring in helping to support assessments of riverine Hg in theMRB in support of the Minamata Conventionon Mercury.