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.
Maakond sai nime Mariposa Creeki järgi, millele omakorda panid nime Hispaania maadeavastajad 1806. aastal, mil nad avastasid suure hulga liblikaid (hispaania ja portugali keeles "mariposas").