Brown issued the declaration Thursday, freeing up funds and firefighting resources for the blaze that has burned more than 84 square miles and is only 2 percent contained.
Other officials had asked for the move, including supervisors in Tuolumne County, who issued a resolution saying the threat posed by the growing fire was beyond their capabilities to handle.
The fire is one of several blazes burning in or near the nation's national parks and has cut off a highway entrance to Yosemite.
It has destroyed two homes and seven outbuildings since breaking out on Saturday.
