Mt Baldy is an unincorporated community in San Bernardino County, California, United States. Mt Baldy is 12 miles (19 km) north of Ontario. Mt Baldy has a post office with ZIP code 91759. The community was established as Camp Baynham in 1906; it changed its name to Camp Baldy in 1910 and became Mt Baldy in 1951. Its post office was established in 1913.