List of the most populous counties in the United States

most populous county11th-most populous12th-most populous14th-most populous15th-most populous16th-most populous county in the nation17th-most populous county in the United States19th-most populous25th-most populous county in the United States2nd largest populated county
This is a list of counties in the United States with a population of more than 100,000 based on the national decennial US census conducted on April 1, 2010 and subsequent mid-2010 and mid-2018 official estimates released by the United States Census Bureau (USCB).wikipedia
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Brooklyn

Brooklyn, New YorkBrooklyn, NYKings
New York County, Bronx County, Kings County, Queens County, and Richmond County, are coterminous with the respective boroughs of New York City, and fall under the jurisdiction of the combined city government.
Brooklyn is a borough of New York City, coterminous with Kings County, located in the U.S. state of New York, the most populous county in the state, and the second-most densely populated county in the United States.

Manhattan

Manhattan, New YorkManhattan, New York CityNew York
New York County, Bronx County, Kings County, Queens County, and Richmond County, are coterminous with the respective boroughs of New York City, and fall under the jurisdiction of the combined city government.
New York County is the United States' second-smallest county by land area (larger only than Kalawao County, Hawaii), and is also the most densely populated U.S. county.

Los Angeles County, California

Los Angeles CountyLos AngelesCounty of Los Angeles
Los Angeles County, officially the County of Los Angeles, in the Los Angeles metropolitan area of the U.S. state of California, is the single most populous county in the United States, with more than 10 million inhabitants.

California

CAState of CaliforniaCalifornia, USA
California also has the nation's most populous county, Los Angeles County, and its largest county by area, San Bernardino County.

Cook County, Illinois

Cook CountyCookCook counties
It is the second-most-populous county in the United States after Los Angeles County, California.

Chicago

Chicago, IllinoisChicago, ILCity of Chicago
Chicago is the county seat of Cook County, the second most populous county in the US, with a small portion of the northwest side of the city extending into DuPage County near O'Hare Airport.

Harris County, Texas

Harris CountyHarrisHarris Counties
As of the 2010 Census, the population was 4,092,459, making it the most populous county in Texas and the third most populous county in the United States.

Maricopa County, Arizona

Maricopa CountyMaricopaMaricopa Counties
The U.S. Census Bureau estimated its population was 4,410,824 as of 2018, making it the state's most populous county, and the fourth-most populous in the United States, containing more than half the population of Arizona.

Miami-Dade County, Florida

Miami-Dade CountyMiami-DadeDade County
According to a 2018 census report, the county had a population of 2,761,581, making it the most populous county in Florida and the seventh-most populous county in the United States.

Dallas County, Texas

Dallas CountyDallas(Dallas County)
It is Texas' second-most populous county and the ninth-most populous in the United States.

Riverside County, California

Riverside CountyRiversideRiverside Counties
As of the 2010 census, the population was 2,189,641, making it the fourth-most populous county in California and the 11th-most populous in the United States.

King County, Washington

King CountyKingWashington
The population was 2,233,163 in the 2018 census estimate, making it the most populous county in Washington, and the 12th-most populous in the United States.

San Bernardino County, California

San Bernardino CountySan BernardinoSan Bernardino Counties
As of the 2010 U.S. Census, the population was 2,035,210, making it the fifth-most populous county in California and the 14th-most populous in the United States.

Tarrant County, Texas

Tarrant CountyTarrantTarrant Counties
It is Texas' third-most populous county and the 15th-most populous in the United States.

Broward County, Florida

Broward CountyBrowardBroward Counties
According to a 2017 census report, the county had a population of 1,951,260, making it the second-most populous county in the state of Florida and the 17th-most populous county in the United States.

Bexar County, Texas

Bexar CountyBexarBexar County Hospital
It is the 16th-most populous county in the nation and the fourth-most populated in Texas.

Wayne County, Michigan

Wayne CountyWayneWayne County Road Commission
As of 2018, the United States Census estimated its population as 1,753,893 making it the 19th-most populous county in the United States.

Palm Beach County, Florida

Palm Beach CountyPalm BeachPalm Beach Counties
According to a 2018 census report, the county had a population of 1,485,941, making it the third-most populous county in the state of Florida and the 25th-most populous county in the United States.

Hennepin County, Minnesota

Hennepin CountyHennepinHennepin County Board of Commissioners
It is the most populous county in Minnesota and the 32nd-most populous county in the United States; more than one in five Minnesotans live in Hennepin County.

Suffolk County, New York

Suffolk CountySuffolkSuffolk County Legislature
As of 2006, Suffolk County ranked at number 21 on the list of the most populous counties in the United States.

Westchester County, New York

Westchester CountyWestchesterWestchester, New York
It is the second-most populous county on the mainland of New York, after the Bronx, and the most populous county in the state north of New York City.

Marion County, Indiana

Marion CountyMarionMarion County Superior Court
Census 2010 recorded a population of 903,393, making it the largest county in the state and 55th most populated county in the country, greater than the population of six states.

Milwaukee County, Wisconsin

Milwaukee CountyMilwaukeeMilwaukee County Board of Supervisors
It is the most populous county in Wisconsin and the 45th most populous in the United States.

Pierce County, Washington

Pierce CountyPierceCharter Amendment One (Pierce County, 2008)
As of the 2010 Census, the population was 795,225, making it the second-most populous county in Washington behind King County, and the 63rd-most populous in the United States.

Snohomish County, Washington

Snohomish CountySnohomishSnohomish Counties
With an estimated population of 814,901 as of 2018, it is the third-most populous county in Washington, after nearby King and Pierce counties, and the 76th-most populous in the United States.