Californians are now required to wear masks in a variety of public settings when social distancing isn't possible. The order by Gov. Gavin Newsom includes visits to high-risk settings such as healthcare facilities and while people are waiting or riding public transit.
It comes one week after Orange County reversed a mask requirement when threats of violence over the mandate forced the county public health officer to step down.
"Simply put, we are seeing too many people with faces uncovered -- putting at risk the real progress we have made in fighting the disease," the Democratic governor said in a statement Thursday.
"California's strategy to restart the economy and get people back to work will only be successful if people act safely and follow health recommendations. That means wearing a face covering, washing your hands and practicing physical distancing."
Other states with mask orders include New York, Illinois, Michigan, Massachusetts, Delaware and Maryland.
"Science shows that face coverings and masks work," Newsom said. "They are critical to keeping those who are around you safe, keeping businesses open and restarting our economy."
The order goes into effect as the state gradually begins to reopen its economy.
It requires that residents wear face coverings when visiting indoor public spaces including lines to enter such places; when visiting hospitals, pharmacies, physician and dental offices; while waiting for or riding public transit, and in outdoor spaces where staying six feet apart from others is not feasible.
The mandate exempts children 2 years old and under, people with medical, mental health or developmental disabilities that prevent them from wearing masks, the hearing impaired and those who communicate with them, and workers for whom masks would violate safety rules.
Other exceptions include restaurant customers who are eating and drinking, and residents during outdoor recreation who keep a distance from others.
Masks are required for food service workers, in parking facilities and hallways, and elevators and stairways.
Former governor Arnold Schwarzenegger tweeted his support for the order.
"This is 100% the right move," he wrote. "This will help us beat this terrible virus. The science is unanimous -- if we all wear masks, we slow down the spread and can reopen safely. It's not a political issue. Anyone making it a political issue is an absolute moron who can't read."
Until now, state health officials had only recommended -- rather than mandated -- the use of masks.
It's unclear how the state will enforce the order.
Protests over mask mandates have turned violent since states began reopening in May.
A town in Oklahoma revoked its mask requirement within a day of enacting it after residents threatened violence. And customers in Michigan are accused of fatally shooting a Family Dollar security guard who told them they needed to wear masks to enter the store.
As Americans continue to re-enter public life this summer, the US Centers for Disease Control laid out recommendations for safety at the beach on Thursday.
Face masks, for instance, should be worn even at the beach (expect when in the water to keep airways clear), beach blankets should be set up 6 feet apart and food, supplies and toys should not be shared, according to the CDC guidelines.
The same guidance about keeping a safe distance from others and wearing a face covering apply to other situations as well, health experts have said.
"Uniform masking would go a long way," Dr. Scott Gottlieb, former commissioner of the US Food and Drug Administration, said during an appearance on the MSNBC program "Morning Joe" Thursday, citing a study that showed the virus could decline rather than continue to expand if only 60% of people wore masks that were 60% effective.