William Alexander Leidesdorff, Jr. was one of the earliest biracial-black U.S. citizens in California and one of the founders of the city that became San Francisco. A highly successful, enterprising businessman, he was a West Indian immigrant of African Cuban, possibly Carib, Danish/Swedish and Jewish ancestry. Leidesdorff became a United States citizen in New Orleans in 1834. He migrated to Alta California in 1841, then under Mexican rule, settling in Yerba Buena, a village of about 30 Mexican and European families.
He became a Mexican citizen in 1844 and received a land grant from the Mexican government, 8 Spanish leagues, or 35,500 acres south of the American River, known as Rancho Rio de los Americanos. He served as US Vice Consul to Mexico at the Port of San Francisco beginning in 1845. Leidesdorff was President of the San Francisco school board and also elected as City Treasurer. Shortly before Leidesdorff's death, vast amounts of gold were officially reported on his Rancho Rio De los Americanos. By the time his estate was auctioned off in 1856, it was worth more than $1,445,000, not including vast quantities of gold mined upon his land.