The expedition led to the founding of Alta California and contributed to the solidification of Spanish territorial claims in the disputed and unexplored regions along the Pacific coast of North America.

Although already inhabited by Native Americans, the territory that is now California was claimed by the Spanish Empire in 1542 by right of discovery when Juan Rodríguez Cabrillo explored the Pacific coast.

In 1596, a Portuguese captain sailing for Spain, Sebastião Rodrigues Soromenho (Spanish: Sebastián Rodríguez Cermeño) explored some of the same coastline, leaving a description of coastal features.

[1][page needed] By the late 1760s, the Spanish king and a handful of other European rulers began to realize the importance the Pacific coast of North America would have in maritime trade and activity.

[additional citation(s) needed] Upon hearing about the king's desire to explore Alta California, New Spain's visitador (inspector general) José de Gálvez organized an exploratory expedition and placed Governor Portolà in overall command.

José de Gálvez, addressing the men waiting to board, declared their final destination as Monterey and their mission to plant the holy cross among the Indians.

Friar Parrón boarded the San Carlos along with captain Vicente Vila, followed by lieutenant Pedro Fages with his 25 Catalan volunteers; cartographer Miguel Costansó, who made maps and drawings to describe the journey; surgeon Pedro Prat; and a crew of 23 sailors, plus two blacksmiths, two boys, four cooks, and mate Jorge Estorace – a crew of 62 in all.

Captain Fernando Rivera, moving north through Baja California, gathered horses and mules from the fragile chain of Catholic missions to supply his overland expedition.

Friar Juan Crespí, selected as chaplain for the Rivera party and diarist for the Franciscan missionaries, traveled for 24 days from Mission La Purísima, approximately 400 miles (640 km) north to Velicatá, then the northern frontier of Spanish settlement in Baja California.

Junípero Serra, assigned by José de Gálvez to head the Franciscan missionary team into Alta California, joined the Portolà party as chaplain and diarist.

[9] Serra, trekking much of the way on a broken-down mule, finally caught up with Portolà, De la Campa and the other members of their party on May 5, just south of Velicatá.

On May 1, lieutenant Pedro Fages, engineer Miguel Costansó, and mate Jorge Estorace came ashore from their anchorage in San Diego Bay, along with 25 soldiers and sailors still healthy enough to work.

[10] Captain Rivera's column arrived on May 14, having trekked 300 miles (480 km) in 50 days from Velicatá[11] without losing a single man or having a sick one – although with their food rations drastically reduced.

[12] The commanding officers prepared to dispatch the San Antonio back to Lower California New Spain, to report to viceroy de Croix and visitador Gálvez about the expedition.

After two weeks of recuperation, Portolà resumed the northward march to rediscover Vizcaíno's port of Monterey by land, with a party of 74 men: lieutenant Pedro Fages with his Catalan volunteers; leather-jacket soldiers; captain Fernando Rivera; sergeant José Francisco Ortega leading the scouts; engineer and cartographer Miguel Costansó; Baja California Christian Indians; and friars Juan Crespí and Francisco Gómez; the Franciscan missionary college of San Fernando had appointed Crespí official diarist of the expedition.

The rest of the party reached Monterey Bay on October 1 – but still failed to recognize it as their destination, because it did not seem to match the grand scale described by Vizcaíno.

Also, Portolà and his hungry men had hoped to find the supply ship San José waiting for them at anchor in their destination harbor of Monterey.

Looking into the Pacific Ocean, they could see the Farallon Islands due west – and Drakes Bay curving broadly to Point Reyes across 40 miles (65 km) of open water to the northwest.

Only friar Crespí seemed to grasp the importance of the bay, describing it in his diary as "a very large and fine harbor, such that not only all the navy of our Most Catholic Majesty but those of all Europe could take shelter in it."

On November 11, Portolà convened an officers' council, which agreed unanimously that 1) they must have passed Monterey, 2) it was time to turn around and retrace their steps back to San Diego, and 3) no one would be left behind hoping for a supply ship to arrive.

On December 10, Portolà ordered his men to plant a large wooden cross where passing ships could see it, with a letter describing the expedition's travels buried at its foot.

Crespí quoted part of the letter: "The cross was planted on a hill on the edge of the beach of the little bay which lies to the south of Point Pinos (pine-covered headland)."

[17] The exhausted men reached San Diego on January 24, 1770 "smelling frightfully of mules", but warmly welcomed by their fellow soldiers and friars.

The new party included Pedro Fages with twelve Catalan volunteers, seven leather-jacket soldiers, two muleteers, five Baja Christian Indians, Portolà's servant, and friar Crespí.

On board were friar Junípero Serra, cartographer Miguel Costansó, and doctor Pedro Prat – along with a stock of supplies for the new mission in Monterey.

Buffeted by unfavorable winds, the San Antonio retreated back south to Baja California, then swung as far north as the Farallon Islands, 100 miles (160 km) northwest of Monterey.

[23] They returned to the wooden cross left on a hill the year before, and this time (perhaps on a clearer day) realized that the site did indeed overlook the place Vizcaíno had described.

[24] On July 9, 1770, Portolà and Costansó boarded the San Antonio and sailed out of Monterey Bay, headed back to Baja California New Spain.

Friendly encounters with the native people had been a goal from the onset of the expedition, and the Spanish brought many items and trinkets with which they traded for supplies and used to create peaceful relations.

When Portolà returned to New Spain in 1770, Pedro Fages (now promoted to captain) was appointed lieutenant governor of Alta California, with headquarters at the Presidio of Monterey.