In the United States, the electricity sector is a major focus for implementing policies to meet national, state, or local mandatory or voluntary CO2 emissions reductions goals. Thus, it is important to have timely and available information on greenhouse gas emissions generated by the power sector to ensure that the policies implemented achieve intended emissions reductions. This work is the first to develop a transparent method to compute the emissions intensity for the US electricity section from 2001 through 2017 at different temporal (annual, quarterly, monthly) and regional (US, NERC, and state) levels. We find that between 2001 and 2017 the average annual CO2 emissions intensity of electricity production in the United States decreased by 30%, from 630 g CO2 kWh−1 to 439 g CO2 kWh−1. This change in CO2 intensity is attributable to an increase in generation from natural gas and wind accompanied by a reduction in coal-fired power generation. The decline in carbon intensity varies across regions, with the largest reduction between 2001 and 2017 from power plants in the Northeast (58%) and the smallest reduction from power plants in the Texas region (27%). In absolute terms the South-central region saw the largest decrease in emissions intensity (358 g CO2 kWh−1) and Texas saw the smallest (164 g CO2 kWh−1). We also find that replacing coal generation with natural gas or renewables has increased the monthly correlation of CO2 intensity between regions. At the state level, Delaware saw the largest decrease in CO2 intensity (466 g CO2 kWh−1), and Idaho is the only state that has not decreased CO2 intensity since 2001.