The mayor of Los Angeles ordered face coverings must be worn at all times outside the home, as the city seeks to contain the coronavirus while it gradually opens back up.
The order came as other restrictions were eased in California's largest city, allowing more people to return to work and exercise outdoors, and more non-essential businesses to restart.
Wearing face coverings "whenever we're away from home will create a meaningful layer of protection for people we might come into contact with, and that makes sense at this stage of our response to the crisis," said Eric Garcetti late Wednesday.
The only exemptions are children under the age of two, and people with certain disabilities.
Until now, wearing face masks in Los Angeles has only been required in shops, public transportation and certain public facilities.
Face coverings have been mandatory in the neighboring city of Beverly Hills as well as nearby Riverside County for several weeks, but those laws have not been strictly enforced.
Los Angeles eased other restrictions, allowing all businesses to offer takeaway sales provided they have direct street access.
Parks and several beaches including Venice Beach have been reopened for physical activity such as walking, jogging and surfing, but not for sunbathing or picnics.
Los Angeles County is the epicenter of the novel coronavirus in California, with more than 34,000 confirmed cases and around 1,660 deaths.