From the Guide to 2010 State and Local Census Geography — California — History:
“The United States acquired the area of California from Mexico in 1848, with generally the same boundary as the present state. The area of California was never organized as a territory, but was administered from 1848 to statehood by a federal military authority. California was admitted to the Union on September 9, 1850, as the 31st state.
“Census data for California are available beginning with the 1850 census; however, the data for 1850 are incomplete because the questionnaires from Contra Costa and Santa Clara Counties were lost and those for San Francisco County were destroyed by fire. Some missing 1850 data have been supplanted by information obtained from a state census in 1852.”
From QuickFacts: statistics for all states and counties, and for cities and towns with a population of 5,000 or more.
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From data.census.gov — the U.S. Census Bureau's new dissemination platform.
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From U.S. Census Bureau History: