Monterey County was one of the original 18 counties considered for creation January 4, 1850 during California's first constitutional convention, held in Monterey.
The state Legislature, after adding an additional 9 counties, officially created Monterey County February 18, 1850. See 1850 County Map for the original county boundaries (Monterey is shown as #28).
In 1874, part of Monterey County was taken to form San Benito County. Otherwise its boundaries have remained the same.