Most Populous Counties In The United States

Los Angeles County has more people than 40 of the 50 states. Not more than 40 cities, more than 40 states. Drop it onto the list of states and it would rank about eleventh, a hair ahead of New Jersey. America's biggest counties are essentially nation-states that happen to hold city-council meetings, and the order reshuffles every year as the Sun Belt swells and the coasts spring leaks. Here are the ten most populous, starting at the very top, using the Census Bureau's July 2025 estimates.

1. Los Angeles County, California (9.69 million residents)

We may as well start with the monster. Los Angeles County is less a county than a small country wearing a county's clothes: about a quarter of all Californians, nearly ten million people, live across its 4,000-plus square miles, in the city of Los Angeles and dozens of others like Long Beach and Pasadena. It has held the top spot since 1970 and is nowhere near giving it up, even after shedding more than 300,000 residents since 2020. It runs its own beaches, mountains, and film industry; if Hollywood's output were a national export, the county would need a trade ministry. The decline is real but relative, when you start this far ahead, losing a midsize city's worth of people barely dents the lead.

2. Cook County, Illinois (5.19 million residents)

Cook County holds more than 40 percent of Illinois inside its borders, so when Chicago sneezes, the whole state reaches for a tissue. Founded in 1831, it bundles Chicago together with suburbs like Evanston and Cicero and a surprising amount of green; the Forest Preserve District protects tens of thousands of acres of woods, prairie, and wetland right up against the city. Cook is the only county in the top three that is shrinking, down about 81,000 since 2020, which is the entire reason Harris County is breathing down its neck. It is still bigger than 27 states, though, so the panic should be measured.

3. Harris County, Texas (5.05 million residents)

Harris County just cracked five million and is gaining on Cook County for the number-two spot, growing while Chicago's county shrinks; give it a few more years and the order may flip. It is, of course, Houston, enormous and humid and home to NASA's Johnson Space Center, where "Houston, we have a problem" was not a movie line but a radio call. Rice University and the University of Houston supply the brainpower, and the Texas Medical Center is the largest medical complex on Earth. The Astrodome, the world's first domed stadium, still stands, mostly empty, a monument to the 1965 conviction that Texas could air-condition baseball.

4. Maricopa County, Arizona (4.69 million residents)

Maricopa is the fastest-growing giant on this list, adding residents at a clip the others can only envy, up more than six percent since 2020. It wraps around Phoenix, the fifth-largest city in the country, plus Tempe, Mesa, and Glendale, all baking happily at 110 degrees. Arizona State University anchors Tempe and routinely ranks among the largest universities in the nation by enrollment. The county is named for the Maricopa people and still contains several tribal reservations. Best of all for the space-minded: cameras orbiting Mars are run from a facility on the ASU campus, which quietly makes Maricopa County a command center for another planet.

5. San Diego County, California (3.28 million residents)

San Diego County packs 70 miles of coastline, a near-perfect climate, and about sixteen military installations into one of the most pleasant places in America to be stationed. The Navy is everywhere; the retired aircraft carrier USS Midway is now a museum whose flight deck you can walk. Tourists get SeaWorld, Legoland, and the cliffs of Torrey Pines. The one thing the county lost was its football team, the Chargers bolted for Los Angeles in 2017 and have been awkward houseguests ever since, though the Padres stayed put. Population barely moved since 2020, which by California standards counts as a victory.

6. Orange County, California (3.15 million residents)

Orange County is what happens when you hand a string of beach towns a theme park and let them marinate. Disneyland opened in Anaheim in 1955 and never left; Knott's Berry Farm down the road began as a literal berry farm with a fried-chicken stand. Richard Nixon was born in Yorba Linda, where his presidential library now sits. And the Crystal Cathedral, the giant glass megachurch, was bought by the Catholic Church and renamed Christ Cathedral in 2012. The Angels still play ball in Anaheim; the county itself slipped slightly since 2020, proof that even paradise loses a few.

7. Miami-Dade County, Florida (2.80 million residents)

Miami-Dade is the rare county where you can canoe through the Everglades and order a Cuban coffee in the same afternoon. It was plain Dade County until 1997, when voters bolted "Miami" onto the front to cash in on the city's fame; the original name honored an Army major killed at the start of the Second Seminole War in 1835. More than half the county's residents were born outside the United States, the highest share of any large county in the country, and Spanish is spoken at home more often than English. It grew nearly four percent since 2020, bucking the coastal-decline trend by importing its growth straight from abroad.

8. Dallas County, Texas (2.66 million residents)

Dallas County just edged past Brooklyn into eighth, which tells you most of what you need to know about where Americans are moving. Note what it does not contain: Fort Worth and Arlington, despite a stubborn myth, sit next door in Tarrant County. What Dallas County does hold is the city of Dallas plus Irving, Garland, and Mesquite, along with a permanent place in the national memory. It was here, at Dealey Plaza, that President Kennedy was assassinated in 1963, and at Love Field that Lyndon Johnson took the oath aboard Air Force One hours later. The county museum in the Old Red Courthouse keeps the era close, down to the cowboy hat J.R. Ewing wore on Dallas.

9. Kings County, New York (2.65 million residents)

Kings County is Brooklyn, the exact same lines on the map, and on its own it would be the fourth-biggest city in America, ahead of Houston. It just slipped a notch here, though, passed by Dallas as New Yorkers kept decamping for cheaper zip codes; the county shed about 82,000 people since 2020. It is also one of the oldest names on this list, organized as a county in 1683, with Dutch roots reaching back to the 1630s. Coney Island still runs its Cyclone, locals still argue Prospect Park out-greens its famous Manhattan cousin, and roughly a third of residents were born in another country.

10. Riverside County, California (2.54 million residents)

Riverside spreads across more than 7,000 square miles of the Inland Empire, which gives it room for both Palm Springs poolside glamour and genuine emptiness. Coachella turns a polo field into the country's most photographed music festival every spring; an hour away sit ghost towns the desert is quietly reclaiming. Out in the scrub near Hemet is Gold Base, a fenced compound with its own film studio that serves as a nerve center for the Church of Scientology. The county grew more than five percent since 2020, which is how a place this dusty keeps climbing the list.

Add up just these ten counties and you get more than 41 million people, around one in eight Americans, packed into a sliver of the nation's land. The pattern in the numbers is hard to miss: the Texas and Arizona counties are surging, the California and New York ones are flat or shrinking, and the country's center of gravity keeps sliding south and west. County lines were drawn for nineteenth-century bookkeeping, but they ended up mapping where modern America actually decided to live. The full top fifty, below, tells the same story in miniature.