Before becoming a county, the Native Americans who were members of the Ohlone Tribe occupied the Alameda land area. For over 3000 years, they lived on the islands cultivating their crops for sustenance. They used tree branches from the willow trees to construct their houses.

The Alameda County bill got approved in March of 1853. The county formed two close territories divided by a creek, Santa Clara County and Contra Costa County. During its formation, the county seat stationed in Alvarado is now a part of Union City.

The county seat was moved to San Leandro three years after Alameda became a county. It was there where the historic Hayward Fault earthquake destroyed the county courthouse, forcing it to get reestablished in Brooklyn, where it still stands today.

Alameda is Spanish for tree-lined paths or groves of poplar trees. The Spanish settlers named the county at the time, who got the idea of the name from the shady trees growing along the creek. The willow and sycamore trees along the river banks resembled a tree-lined walk to the early Spanish explorers. While "Poplar Grove Creek" is a literal English translation, the mainstream that runs through the county is now known as "Alameda Creek."