The United States has seen large improvements in air quality over the last half century with the implementation of regulations designed to reduce air pollutant emissions. Regulatory costs, estimated by the Environmental Protection Agency at tens of billions of dollars per year, motivate air pollution accountability research, which evaluates impacts of air quality regulations on emissions, air quality, exposure/dose, and public health—components of the so-called Accountability Chain. This work conducts a detailed analysis of a range of regulatory actions on electricity generating units and on-road mobile sources promulgated since the 1990s from the action at the federal level, implementation at the state level, the resulting emissions changes and the impacts on air quality and health. Results show that the United States has seen major emissions reductions over this period attributable to regulatory policies, although influences such as fuel costs, demographic shifts, and technological improvements have influenced emissions reductions as well. The resulting emissions reductions have led to air quality and health benefits.