This policy brief analyzes the effects of COVID-19 mitigation policies, those that restrict movement and activity and those that advocate public health best practices. The analysis uses US state-level data to estimate the effects of mobility, mask mandates, and compliance with these mandates on the numbers of COVID-19 cases and deaths. A one-standard-deviation increase in mobility is associated with an 11 to 20 basis points greater rate of growth in case counts; a mask mandate can offset about half of this increase. Slower growth in case counts ultimately translates into slower growth in death counts. Mask mandates are more effective in states where compliance with those mandates is higher. Our estimates imply that total infections in the United States would have been 46.5 to 66.2 percent lower than they were on November 15 if mobility had remained fixed at its May 15 level. Given the actual mobility level, if a national mask mandate had been enacted on May 15, the case count would have been 26.4 to 34.3 percent lower than it was on November 15. This means that a national mask mandate potentially could have offset as many as 74 percent of the additional COVID-19 cases associated with increases in mobility.