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 great clusters of butterflies ("mariposas" in Spanish) in the foothills of the Sierras. For many years, residents marked the annual arrival (in May) of migrating Monarch butterflies with a "Butterfly Days" parade.