Mariposa County was one of the original 27 counties of California, created at the time of statehood in 1850. While it began as the state’s largest county, territory that was once part of Mariposa was ceded over time to form all or part of twelve other counties, including all of Merced, Madera, Fresno, Tulare, Kings, and Kern; and parts of San Benito, Mono, Inyo, San Bernardino, and Los Angeles. Thus, Mariposa County is known as the “Mother of Counties”.

Mariposa County’s original seat was a now-nonexistent hamlet known as Agua Fria (Spanish for “cold water”), about 3 miles directly west of Mariposa proper on Agua Fria Road, which runs from Highway 140 to the south, to the community of Mt. Bullion to the northwest. Charles Fremont moved the county seat to Mariposa in 1854, resulting in the construction of the Mariposa County Courthouse, whose grounds occupy an entire block. The historic structure is fronted by Bullion Street; Jones Street is to the rear, with 9th and 10th Streets on either side. The courthouse’s likeness is on the Mariposa County Seal.

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 Sierra. Each year, the first weekend in May, residents mark the annual arrival of migrating monarch butterflies with a “Butterfly Days” festival and parade.