According to this article, fourteen states and the District of Columbia enacted statewide mandates before June 1st requiring masks to be worn in public. Looking at data for June, we see that states which had a mandate fared much better than those states that did not. See the graph below.
These are huge differences. Since June 1st, an additional nine states have ordered residents to wear masks in public. That is still less than half the states. If we want to control the spread of COVID-19 in the US, we need many more states to join this list. This includes Alabama, Florida, Georgia, Tennessee, and pretty much every other state in the southeast except for Kentucky and Virginia.