The Mission Vieja or Misión Vieja or the Old Mission was the first Spanish mission in the San Gabriel Valley. Mission Vieja was built in 1771 by what would become the fathers of the Mission San Gabriel Arcángel. The Mission Vieja site was designated a California Historic Landmark (No.161) on Jan. 11, 1935. The location of Mission Vieja is at what is now the South West corner North San Gabriel Blvd and North Lincoln Ave in Montebello, California. The site has a plaque marker and an El Camino Real Bell. Mission Vieja was the first building in the San Gabriel Valley and Los Angeles County. Father Juan Crespí picked the spot for Mission Vieja because water was available at what is now called the Whittier Narrows. The place for Mission Vieja was found in the expedition of Portolá in the summer of 1769. Gaspar de Portolá, Miguel Costansó and Father Juan Crespí recorded the expedition. In 1771 the fathers of Pedro Cambon and the fathers of Angel Somera, with a decree from Governor Pedro Fages, set out to build a mission in the Spanish Empire of Alta California, in what is now called the Los Angeles Basin. The first building was 45 by 18 feet made of adobe and wood. The roof was made with tule reeds from the waters of the Whittier Narrows. The building had around it a weak Palisade fence. Father Angel Somera and Father Pedro Cambon, both Franciscan missionaries, founded Mission Vieja, the original Mission San Gabriel Arcangel, on September 8, 1771; the location is today near the intersection of San Gabriel Boulevard and the Rio Hondo River. The establishment of the mission marked the beginning of the Los Angeles region's settlement by Spaniards and the fourth of twenty-one missions ultimately established along California's El Camino Real. The mission did well initially as a farm and cattle ranch. Six years after its founding, however, a destructive flood in 1776 led the mission fathers to relocate the mission five miles north, to its current location in what is the present day City of San Gabriel.

The Whittier Narrows is a narrows or water gap in the San Gabriel Valley of Los Angeles County, California, United States, between the Puente Hills to the east and the Montebello Hills to the west. The gap is located at the southern boundary of the San Gabriel Valley, through which the Rio Hondo and the San Gabriel River flow to enter the Los Angeles Basin. The Narrows is located near the convergence of Interstate 605 and California State Route 60.