Brown issued the emergency declaration on Saturday, saying the state's extreme drought has made fire conditions particularly dangerous.
The declaration called for the state's National Guard to mobilize in response to the blazes.
California is in the midst of one of its most severe droughts on record, a factor that fire officials say has contributed to the growing number and intensity of wildfires across the state this year.
The declaration allows various state agencies to cooperate while battling at least 14 significant fires currently burning in California, including the Oregon Gulch fire, which started in Oregon last week then spread over the border.
That blaze has charred about 32,000 acres; 23,000 in Oregon and the rest across the border in California, according to the California Department of Forestry and Fire Protection.
The blaze was expanding rapidly amid extremely hot, dry conditions, said CalFire spokesman Dennis Mathisen. "This is the third really dry year we've had," he said. "The fire behavior we are seeing out there is usually not behavior we see until September," he said.
The Oregon Gulch blaze, part of the larger Beaver Complex, has destroyed three homes and five outbuildings, and was threatening another 280 dwellings, fire officials said.
Evacuation notices were in effect for several communities on both sides of the border.
The more destructive of the twin fires was threatening the town of Burney, where officials at Mayer Memorial Hospital decided to evacuate their 49-bed annex for patients with dementia and other conditions requiring skilled nursing. The patients were transferred to a hospital in Redding, about 55 miles away, the hospital reported on its website.
About two dozen fires are currently burning from California to Washington state, many triggered by lightning and then fanned by hot temperatures and strong winds.
"With the current conditions, all you need is a spark," said Mathisen. "We know what the typical fire season should look like. This year nothing's typical."
Elsewhere in California, thunderstorms across Southern California on Sunday brought flash floods that led to a few rescues, while thick debris flows cut off access to two mountain towns and stranded more than 2,000 people people.
About 1,500 residents of Oak Glen, and another 1,000 residents of Forest Falls in the San Bernardino Mountains were unable to get out because the roads were covered with mud, rock and debris, authorities said.
The stranded include 500 people who had arrived at a Forest Falls campground Sunday morning.