Reductions in greenhouse gas (GHG) emissions can bring ancillary benefits of improved air quality and reduced premature mortality, in addition to slowing climate change. Here we study the co-benefits of global and domestic GHG mitigation on US air quality and human health in 2050 at fine resolution using dynamical downscaling of meteorology and air quality from global simulations to the continental US, and quantify for the first time the co-benefits from foreign GHG mitigation. Relative to the reference scenario from which Representative Concentration Pathway 4.5 (RCP4.5) was created, global GHG reductions in RCP4.5 avoid 16 000 PM2.5-related all-cause deaths yr(-1) (90% confidence interval, 11 700-20 300), and 8000 (3600-12 400) O-3-related respiratory deaths yr(-1) in the US in 2050. Foreign GHG mitigation avoids 15% and 62% of PM2.5-and O-3-related total avoided deaths, highlighting the importance of foreign mitigation for US health. GHG mitigation in the US residential sector brings the largest co-benefits for PM2.5-related deaths (21% of total domestic co-benefits), and industry for O-3 (17%). Monetized benefits for avoided deaths from ozone and PM2.5 are $137 ($87-$187) per ton CO2 at high valuation and $45 ($29-62) at low valuation, of which 31% are from foreign GHG reductions. These benefits likely exceed the marginal cost of GHG reductions in 2050. The US gains significantly greater air quality and health co-benefits when its GHG emission reductions are concurrent with reductions in other nations. Similarly, previous studies estimating co-benefits locally or regionally may greatly underestimate the full co-benefits of coordinated global actions.