The order says those attending "outdoor mega-events" must wear a face covering at all times except when eating and drinking.
The new mandate is an addition to existing orders issued last month which require masks while entering most public indoor spaces, including major indoor events of 5,000 or more, as well as bars, restaurants and gyms.
The county had previously exempted outdoor mega events from that order.
An outdoor mega event is defined as a gathering of more than 10,000 people outdoors in one location, such as sporting events, marathons, festivals, concerts and car shows. The operators of the events are responsible for making sure patrons follow the mask rules, the county says.
Outdoor venues that had indoor seating areas or concourses, like Dodger Stadium, have been requiring masks be worn while in those areas, but until now most had not required them in outdoor seating areas.
Sofi Stadium is a hybrid of indoor and outdoor, with a roof covering but open air on the sides. Since last month's county mask order, the Inglewood football stadium has been requiring masks in indoor areas such as concourses and restrooms but not in most seating areas.
Amid a surging Delta variant among the unvaccinated, Los Angeles County reported 2,426 new COVID-19 cases on Monday as well as five additional deaths.
Recent data show about 67% of Los Angeles County's eligible population is vaccinated. Orange County announced that 75% of its eligible population has received at least one dose of vaccine.
But there is growing concern for unvaccinated children. A new study finds younger children are more likely to spread the virus to family members in their homes. Health officials say masks in schools will help.