Mariposa County was one of the original counties of California, created at the time of statehood in 1850. While it began as the state's largest county, over time territory that was once part of Mariposa was ceded to twelve other counties: Fresno, Inyo, Kern, Kings, Los Angeles, Madera, Merced, Mono, San Benito, San Bernardino, San Luis Obispo, and Tulare. Thus, Mariposa County is known as the "Mother of Counties".
The county took its name from Mariposa Creek, which was so named by Spanish explorers in 1806, when they discovered a great cluster of butterflies ("mariposas" in Spanish and Portuguese) in the foothills of the Sierras. Each year, the first weekend in May, residents mark the annual arrival of migrating Monarch butterflies with a "Butterfly Days" festival and parade.
Mariposa County is located at the southern end of California's Mother Lode region. During the California Gold Rush, great quantities of the prized mineral were found and extracted, first in local stream-beds and later in hard rock mines. One of the most notable beneficiaries of this wealth was the famed explorer and 1856 Republican presidential candidate, John C. Frémont, for whom the local hospital is named. (Jessie Street, in the town of Mariposa, is named for Fremont's wife, Jessie Benton Frémont, who came to Mariposa to live with him.)
Many aspects of the area's mining history are depicted in exhibits at two local museums the Mariposa History Museum, located in the town of Mariposa; and the California Mining and Mineral Museum, located at the Mariposa Fairgrounds (2 miles east of Mariposa on Highway 49). (Photo & text from Wikpedia).