Arctic sea ice regulates the air−sea exchange of volatile mercury (Hg) species like dimethylmercury (DMHg) or elemental Hg and is known to host Hg methylating microbes that produce neurotoxic and biomagnifying monomethylmercury (MMHg). Arctic sea ice accounts for 57% of the total primary production in the Arctic Ocean, suggesting that it could be the main source of MMHg to arctic food webs. Despite this, little is known about Hg concentrations and speciation in arctic sea ice. Here, we report Hg species and show the importance of sea-ice composition on sea-ice methylmercury (MeHg = DMHg + MMHg) budgets. We propose that the shift from older sea ice (lower MeHg) to younger sea ice (higher MeHg) resulted in a 40% increase in MeHg (per square meter of sea ice) since 1979 despite a 45% decline in the total sea-ice volume. Approximately 30% of the MeHg sea-ice budget is DMHg, which means that when the seaice melts in the summer it could contribute 0.03−2.7 pmol of DMHg m⁻² day⁻¹ to the atmosphere, which is comparable to diffusion from water (0.48−2.8 pmol of DMHg m⁻² day⁻¹). This study shows that the MeHg content of rapidly shrinking arctic sea ice and exposure of sea-ice biota may not be declining as previously thought.