Mount San Antonio, commonly referred to as Mount Baldy or Old Baldy, is a 10,064 ft summit in the San Gabriel Mountains on the border of Los Angeles and San Bernardino counties of California. Lying within the San Gabriel Mountains National Monument and Angeles National Forest, it is the highest point within the mountain range, the County of Los Angel…Mount San Antonio, commonly referred to as Mount Baldy or Old Baldy, is a 10,064 ft summit in the San Gabriel Mountains on the border of Los Angeles and San Bernardino counties of California. Lying within the San Gabriel Mountains National Monument and Angeles National Forest, it is the highest point within the mountain range, the County of Los Angeles, and the Los Angeles metropolitan area.