Los Angeles County's original boundaries, when California first became a state, included present-day Orange County and portions of present-day Ventura and Riverside Counties.
For a brief time, Los Angeles County's boundaries were considerably extended to absorb portions of neighboring Mariposa and San Diego Counties. It created the largest extent of the county, reaching as far as California's current state line with neighboring Nevada and Arizona. Then, Los Angeles County included all of the present-day counties of Orange and San Bernardino and portions of present-day Ventura, Riverside, and Kern Counties.
A year after Los Angeles County's greatest expansion, extending all the way to the state line with present-day Nevada and Arizona, the eastern portion of the couty was split off to form the new San Bernardino County.
Los Angeles County's boundaries contracted further after a northern portion became part of the new Kern County.
After a portion of Los Angeles County became part of the new Ventura County in 1873, Los Angeles County had been reduced to its present-day territory and all of present-day Orange County.
Los Angeles County was further reduced in size when its southern portion was split off to form the new Orange County.
Since 1889, only very small adjustments were made to Los Angeles County's boundaries. The most recent was in 2001 when a small portion of northwestern Los Angeles County was transferred over to Kern County so that residents there would be more conveniently served by Kern County. In 2005, residents of the neighboring unincorporated community of Gorman also sought approval to transfer their community to Kern County, however, their efforts were not successful.