Deep, rapid cuts in greenhouse gas emissions are needed to limit future global temperature increases to 1·5°C above pre-industrial levels, but current progress is inadequate to achieve the goals of the Paris Climate Agreement and to reduce future risks from climate change. Many actions to mitigate greenhouse gas emissions can also deliver near-term health co-benefits, for example from reduced air pollution, consumption of healthy diets, and increased physical activity. High-quality evidence on the type and magnitude of co-benefits that can be realised and improved knowledge of how to promote the implementation of such actions can support progress towards net-zero greenhouse gas emissions by 2050. The Lancet Pathfinder Commission was established to collate and assess the evidence on the near-term health effects of greenhouse gas mitigation, including both modelling studies and evaluated implemented actions. The Commission's aim is to assess the potential and achieved magnitude of the benefits for health and climate of different mitigation actions and, where possible, the factors facilitating or impeding implementation.