What is now California was first settled by various Native Californian tribes before being explored by a number of European expeditions during the 16th and 17th centuries. The Spanish Empire then claimed it as part of Alta California in their New Spain colony. The area became a part of Mexico in 1821 following its successful war for independence but was ceded to the United States in 1848 after the Mexican -- American War. The western portion of Alta California then was organized and admitted as the 31st state on September 9, 1850. The California Gold Rush starting in 1848 led to dramatic social and demographic changes, with large - scale emigration from the east and abroad with an accompanying economic boom.
when did california become part of the us
on September 9, 1850