When he returned to his land, he found his cattle had been stolen and was forced to sell his land. He then formed a partnership with John Sutter to build a sawmill. It was at the sawmill that, on January 24, 1848, he discovered gold in the water flow through the mill’s tail race. He immediately advised Sutter, who swore all his employees to secrecy. But, the "news" was just too big, and in no time it leaked out.

As word quickly spread, some 80,000 miners flooded the area, extending up and down the length of the Sacramento Valley, and overrunning Sutter’s domain. Ironically, neither Sutter nor Marshall ever profited from the discovery that should have made them independently wealthy. Though Marshall tried to secure his own claims in the gold fields, he was unsuccessful. His sawmill also failed, as every able-bodied man took off in search of gold. Soon, the area surrounding Sutter's Mill became the first mining boom town in California - Coloma.

Embittered, Marshall left the area, drifting from place to place in California, looking for yet another rich strike. In 1857, he returned to Coloma and started a vineyard in the early 1860's. Initially profitable, his endeavors as a vintner would also fail when, by the end of the decade, increased competition and less demand put him out of business once again.

Marshall then returned to prospecting and wandering about the state. He soon partnered up with another miner in a gold mine near Kelsey, California. However, the development of the mine proved expensive and yielded nothing, leaving the unlucky Marshall once again close to bankruptcy.

In 1872, Marshall had a turn of luck when the California State Legislature awarded him a two-year pension in recognition of his role in an important era in California history.

The pension was renewed in 1874 and 1876, but lapsed in 1878. According to the legend, Marshall then went to visit the legislature assembly in person to get the pension renewed again. However, when a brandy bottle dropped from his pocket and rolled on the floor, no additional pension was awarded.