In 1845, a number of New York Mormons decided to relocate to California. Brannan led a group of about 200 Mormons around South America, to the Hawaiian Islands, and eventually to the small settlement of Yerba Buena, which is now San Francisco. At the time, California was still a Mexican territory. Correspondence from Brannan would later suggest that he felt Brigham Young sent him to California to get him out of the way or to possibly get him killed.
Brannan didn't remain in Yerba Buena for long. He soon moved to John Sutter's settlement near the Sacramento River, where he opened a general store. The store became very successful, but also a source of controversy; Mormons claimed the store was built with tithes which Brannan had taken from the church. When Brannan refused to return the money, he was forced out of the church. He reportedly told church officials that he would return the Lord's money when he got a signed receipt from the Lord.
Brannan took advantage of the discovery of gold in 1848 by buying all the mining tools he could find, then walking through the streets shouting the news that gold had been discovered. He then sold his tools at a huge mark-up to the miners who flooded into the area. Within a few years, Brannan was the state's first millionaire.
In the 1850s, Brannan returned to San Francisco and continued to grow his fortune. He also became involved in local politics. Like many Gold Rush stories, however, Brannan's did not end well. He made a vast amount of money in his life, but excessive drinking, a costly divorce, and a volatile temper caused him to lose his fortune. When he died in 1889, his body lay unclaimed in a morgue for more than a year before being recognized and given a proper burial.