Anglosaxon migrants were trickling into Alta California. First and foremost, there was the Canadian Hudson's Bay Company, which in 1824 had built its Pacific outpost, Fort Vancouver, on the Columbia River. The Mexicans, like the Spaniards before them, had little interest in the north and in the interior of Alta California, which allowed the company's hunters and trappers to roam the rivers of southern Oregon and northern California. In 1826 Alexander McLeod started exploring and trapping in northern California and in 1829 he reached the Sacramento River Valley thereby linking the Columbia River (Fort Vancouver, in today's Washington State) with California's Central Valley (today's Sacramento) in what became known as the Siskiyou Trail (today's Interstate 5 largely follows the Siskiyou Trail).

While searching for a mythical river, St Louis fur trapper Jedediah Smith became the first US citizen (and possibly the first Caucasian man ever) to cross the Mojave desert into Mexico's Alta California, reaching San Gabriel Mission in November 1826, the first to cross the Sierra Nevada east into Nevada (1827), and the first to reach Oregon Country overland from California (1828).

The wealthy fur trader Ewing Young had pioneered the connection of California with the lucrative Santa Fe Trail, a route for horses between Louisiana (notably St Louis) and the Mexican border (Santa Fe, the northern terminus of El Camino Real de Tierra Adentro, which led to Ciudad de Mexico), a route created over the decades by Spanish and French traders and improved for wagon traffic in 1821 by William Becknell just weeks before Mexico's independence opened up commerce for US traders: in 1830 Young set out from Santa Fe and followed Jedediah Smith's route to move to California, where he fur-hunted for a while. In 1834 he started plying the Siskiyou Trail to sell horses, mules and cattle to Oregon Country, where he eventually settled down.

The first industry of California was therefore fur trading.

The first US citizens to build fortunes in Alta California were the sea merchants of the 1830s. Alpheus Thompson, who had started his career in Canton in 1821 and in Hawaii in 1825, settled in Santa Barbara in 1834 and got rich trading with China. Faxon Atherton started his business in 1836 with ships plying between Yerba Buena and San Diego, and then made a fortune in Chile. Atherton settled in 1858 in Valparaiso Park in San Mateo County (the town that grew out of that "park" was eventually named Atherton and became one of the richest towns in the USA). William Heath Davis, born in Hawaii, settled in Yerba Buena in 1838, erected the city's first brick building in 1849, and in 1850 he founded New Town San Diego. Thomas Larkin settled in 1832 in Monterey and quickly became the wealthiest person in town (and therefore one of the wealthiest in all of Alta California). His illegitimate son Oliver was possibly the first white Anglosaxon child born in California (not of mixed race). When gold was discovered, Larkin moved to San Francisco in the second brick building of the city. The second industry of California was therefore maritime trade.

John Marsh, who had made money but also created enemies with his trade with the "Indians", immigrated to California in 1836 via the Santa Fe Trail and became a Mexican citizen in order to buy in 1837 a ranch in Contra Costa (today's area north of Berkeley). From the beginning, Marsh plotted against Mexico: he conducted a campaign to convince US citizens to emigrate to California, hoping that a flood of Anglo-Saxons would lead to a Texas-style secession from Mexico.

In 1839 Johann "John" Sutter, a Swiss immigrant wanted for debts in his home country who had arrived five years earlier in New York, and who one year earlier had travelled with missionaries to Oregon Country spending time also in Russian Alaska, obtained land on the American River (in today's Sacramento) from the Mexican governor of Alta California, Juan Bautista Alvarado, and in 1841 erected a fort, meant as the first building of a utopian “New Helvetia” which quickly became a plantation-style colony modeled after the Caribbean ones, relying on imported Hawaiian ("Kanakas") workers and enslaved Indios. A Hawaiian-born employee was William Heath Davis, the son of a Boston trader and a Hawaiian princess, who had moved to Yerba Buena in 1838 and guided Sutter up the Sacramento River in 1839 to scout a location for his fort. They picked a location near the confluence of the Feather, American Fork, and Sacramento rivers. By 1846 it became the main trading post in California, a place where trappers, Indians, soldiers and immigrants (like Heinrich Lienhard, a fellow Swiss) left behind the wilderness and congregated.

The third industry of California was therefore agriculture. In 1841 the 21-year-old John Bidwell, a native of New York, led the first wagon train of pioneers out of Missouri across the mountains into eastern California (in six months). One of the "passengers" was the German immigrant Carl Weber. The route was later improved and simplified after the discovery of gold. A group of 12 men and their wives and children turned north to Oregon and de facto initiated the Oregon Trail.

Meanwhile, Oregon Country was experiencing a surge of settlers. In 1834 a group of priests (notably the missionary Jason Lee) and fur trappers (notably Nathaniel Wyeth, a wealthy Boston merchant who was trying to compete with the Hudson's Bay Company) traveled from Missouri to Oregon via the still unnamed Oregon Trail. Wyeth also founded Fort Hall, which would become the place where the Oregon and the California trails forked. That expedition set the trend: afterwards, immigrants of Oregon Country were mostly trappers like Joseph Meek, who reached Oregon in 1840 (the first wagons to reach the Columbia River overland) and who in 1841 served as the scout for the United States Exploring Expedition, but also missionaries like Marcus and Narcissa Whitman, who founded a mission in 1836 and returned in 1842 leading a wagon train along the Oregon Trail (they were famously killed by the Cayuse in 1847). William Gray, who had originally arrived with the Whitmans, later wrote the book "A History of Oregon, 1792-1849".

The United States Exploring Expedition, dispatched by US president Andrew Jackson, consisted of seven ships (notably the glorious USS Vincennes) commanded by Charles Wilkes. It explored the Pacific Ocean between 1838 and 1842 carrying a crew that included several scientists and cartographers.

Somehow the popularity of the Far West (Oregon Country and Alta California) increased rapidly, despite the fact that reaching it required several months of rough travel (whether by sea via Cape Horn or overland via the Oregon Trail) and that neither Oregon nor California belonged rightfully to the USA. Both politicians and journalists championed the idea of "manifest destiny", that God had meant the USA to extend from one coast to another. Oregon Country experienced a "great migration" in 1843 (actually involving only about a thousand immigrants), mainly to Willamette Valley (today's Salem) so that in 1843 there were enough Anglosaxon and Francophone settlers to create a provisional government.

An Oregon resident, Lansford Hastings, fell in love with Alta California in 1843 and started dreaming of an independent Republic of California. In 1845 he published a guide book titled "The Emigrants' Guide to Oregon and California" which encouraged migration along the vastly improved Oregon Trail, especially to California, and it managed to convince quite a few people. In 1846 a wagon train of pioneers, later known as the "Donner Party", followed Hastings' book but got lost in the Sierra Nevada where many died of starvation (the survivors, including the Prussian Louis Keseberg who later settled at Fort Sutter, had notoriously resorted to eating the bodies of those who had died). In July 1846 a group of 238 Mormons escaping religious persecution landed at Yerba Buena, led by Samuel Brannan, the highest-ranking Mormon in New York who had printed Mormon newspapers (and promoted Hastings' book in the Mormon community). When they had left New York in January, Alta California was still a province of Mexico, when they arrived via Cape Horn in July, it had been taken over by US-born rebels and Yerba Buena was now called San Francisco. This was the time when Brigham Young was leading 15,000 Mormons on an overland trail to the Great Salt Lake, the prehistory of Mormon Utah. Brannan's Mormons tripled the population of tiny San Francisco. Sam Brannan quickly left the Mormon Church and became a businessman. In January 1847 he launched San Francisco's first newspaper, the weekly California Star, and by the end of 1847, he was running a store at Sutter's Fort.

The first detailed maps of California and Oregon were produced by the army officeer John Fremont, who led a group of topographers and cartographers in three expeditions from 1842 to 1846 on behalf of the US army (his guide was Kit Carson). During the second expedition of 1844 he discovered the Old Spanish Trail used by Mexican traders and named it that way.

Meanwhile, the Mexican population was still mostly concentrated around the missions. While the population of Alta California was still tiny, there was constant turmoil. In 1831 wealthy landowners of Los Angeles and San Diego rebelled against the governor, the first of several rebellions. Until 1834 most of the useful land of Alta California was controlled by the missions, i.e. by the Catholic Church. In 1834 Mexico enacted the "secularization laws" that confiscated those lands and distributed them to lay Mexican-born settlers. For example, the Mission San Francisco Solano (in today's Sonoma) controlled about 4,000 square kilometers. The Mexican government sent the Monterrey native Mariano Guadalupe Vallejo to secularize this mission and he took advantage, becaming the richest man in California. He took Rancho Petaluma for himself and was ordered to found the Pueblo de Sonoma. Juan Bautista Alvarado y Vallejo, who too was born in Monterrey itself, the son of Vallejo's sister, received Rancho El Sur on the Big Sur coast. Another powerful family, the Castros, was based in Villa de Branciforte. The patriarch's daughter, Martina Castro, became the first woman in California to obtain a land grant (in present-day Capitola, near Santa Cruz).

The ranchos simply replaced the missions continuing with the same agriculture and keeping the same "Indians" as laborers.

In 1836 the Mexican government enacted laws that limited the power of provinces and instead centralized power. Inspired by Texas, Alvarado and Vallejo conspired with people from the USA to declare the independence of Alta California from Mexico. The secession didn't succeed but Alvarado successfully negotiated to become governor instead. He was from the San Francisco Bay area. In 1844 Alvarado again conspired against the Mexican government, which had replaced him with a new governor, and after a couple of battles near Los Angeles (between tiny armies which included immigrants from the USA), Pio Pico, born at Mission San Gabriel Arcangel near Los Angeles, was installed as governor. There was rivarly between the south of Alta California and the north: different prominent families competed for control of the government.

Immigration was still scarce. Not many people wanted to move into this distant and primitive place. In 1840 there were about 8,000 Mexicans in Alta California, the so-called "Californios". The Pueblo of Los Angeles had become the largest urban settlement (about 1,500 people) but most Californios were spread out across the 455 ranchos of Alta California. We don't know how many "Indians" lived in Alta California. There were hundreds of immigrants from the USA, like John Sutter and John Marsh, who now owned large ranches (both Johns were involved in political intrigues and military adventures).