El Camino Real de Alto California, (English: Royal Road of Upper California). On this third artery of the Los Caminos Reales, on the North American southwest, the road took a turn westward to reach beyond the deserts to the Gila River at a point just before it arrived at the Colorado River. Here, not far from the Colorado, the Spanish expeditions established settlements. From there, Spanish soldiers, settlers, and missionaries invaded the homelands of the Indigenous peoples of California, people of the Great Basin, and the Pueblo peoples as these invaders began the establishment of the Alta California province of New Spain.

This road has as its destination the areas of Upper California, also known as New California among other names, and was a province of New Spain formally established in 1804. Along with the Baja California peninsula, the province had previously comprised the province of Las Californias, but was made a separate province in 1804. Following the Mexican War of Independence, it became a territory of Mexico in April 1822 and was renamed Alta California in 1824.

During the control of Upper California by Mexico, this territory included all of the modern day states of California, Nevada, and Utah, as well as parts of Arizona, Wyoming, and Colorado. In 1836, Upper California territory was re-combined with Baja California, as a single territory by Mexico in the Seven Laws constitutional reform, which also granted the territory more autonomy. However, that change was subsequently undone in 1846, but rendered moot by the outcome of the Mexican–American War in 1848, when most all of the areas formerly contained within Upper California were ceded to the United States by treaty at the end of the war. In 1850, California joined the union as the 31st state.

Pacific Highway 1 parallels the old highway

Between 1683 and 1835, Jesuit and Franciscan missionaries established a series of religious outposts from Baja California and Baja California Sur into present-day California. Heavy freight and long distance passenger movement was practical only via ships by a coastal water route.

To help encourage overland travel, mission settlements were approximately 30 miles (48 km) apart along the 600-mile long El Camino Real so that each mission would only be separated by one long day of riding on horseback. A story has been told that the missionaries as they traveled between the missions, they would sprinkle mustard seeds along the trail to mark it with bright yellow flowers, creating a golden trail stretching from San Diego north to Sonoma just above San Francisco.

By the mid 1800s, after California became a state, the route had been improved in certain sections, but was still not entirely adequate for large stagecoaches and freight wagons. Then, in 1892, Anna Pitcher of Pasadena, California initiated an effort to establish a commemorate route along the missions in California, which was adopted by the California Federation of Women′s Clubs in 1902.