Sonora is the county seat of Tuolumne County, California, United States. Founded during the California Gold Rush by Mexican miners from So…Sonora is the county seat of Tuolumne County, California, United States. Founded during the California Gold Rush by Mexican miners from Sonora, the city population was 5,003 during the 2020 Census, an increase from the 4,610 counted during the 2010 Census.