researchers at RationalGround.com, a clearinghouse of COVID-19 data trends run by a grassroots group of data analysts, computer scientists, and actuaries, did an analysis of all 50 states divided by those that had mask mandates and those that did not. Justin Hart, co-founder of the website, posted the results in a Twitter thread and shared with me the data analysis:

They studied the number of cases over a 229-day period from May 1 through Dec. 15 and divided the results of the two study groups by days with mask mandates and days without mask mandates. The non-mandate data group includes both states that never had a mandate and those that did at some point, but data set included only the days they did not have a mask mandate.