High latitude ecosystems are prone to phenological mismatches due to climate change- driven advances in the growing season and changing arrival times of migratory herbivores. These changes have the potential to alter biogeochemical cycling and contribute to feedbacks on climate change by altering greenhouse gas (GHG) emissions of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) through large regions of the Arctic. Yet the effects of phenological mismatches on gas fluxes are currently unexplored. We used a three-year field experiment that altered the start of the growing season and timing of grazing to investigate how phenological mismatch affects GHG exchange. We found early grazing increased mean GHG emission to the atmosphere despite lower CH₄ emissions due to grazing-induced changes in vegetation structure that increased uptake of CO₂. In contrast, late grazing reduced GHG emissions because greater plant productivity led to an increase in CO₂ uptake that overcame the increase in CH₄ emission. Timing of grazing was an important control on both CO₂ and CH₄ emissions, and net GHG exchange was the result of opposing fluxes of CO₂ and CH₄. N₂O played a negligible role in GHG flux. Advancing the growing season had a smaller effect on GHG emissions than changes to timing of grazing in this study. Our results suggest that a phenological mismatch that delays timing of grazing relative to the growing season, a change which is already developing along in western coastal Alaska, will reduce GHG emissions to the atmosphere through increased CO₂ uptake despite greater CH₄ emissions.