The 9 Most Populated Counties in the US

The most populated counties in the US tend to center around major cities. Since millions of people fit into just one city, they can hold a good percentage of the total county’s population, skewing numbers.

That doesn’t mean that the most populated counties never change. Over time, populations have shifted. More and more every year, people are moving to the west and the south. This means that states and counties within them in the north and east have been slowly losing population. 

Additionally, remote work has enabled people to live in smaller towns around the US instead of major cities. However, despite all of this, the two most populous counties in the nation have held steady in their titles. 

To learn what the two most populous and steady counties are, as well as the other counties trailing behind, read through the list below.

1. Los Angeles County, California

Los Angeles County, California is home to an estimated 10 million people. The country stretches out over 4,057 miles, actually making it one of the largest counties in the US as well as the most populated. This means that there are roughly 2,465 people per square mile throughout the county.

The county seat is in Los Angeles, the county was founded along with the state in 1850. Besides this major city, there are 87 other towns and cities in the county. Roughly a quarter of the population of California lives in Los Angeles County.

2. Cook County, Illinois

Located in Illinois, the total population of Cook County is home to 5.27 million people. This county covers 945 miles, which means the population per square mile is about 5,583.

The reason for so many people in the county comes down to the one major city included in the area. Chicago, which is the county seat, has roughly half of the total population of the county.

3. Harris County, Texas

Harris County is located in the southeast part of Texas. 4.7 million people are spread out across 1,703 miles. This places it on par with Los Angeles County with 2,760 people per square mile.

The county seat is Houston. Interestingly, Over 83 percent of the population in Harris County was born and raised in the county. Originally, in 1836, it had the name Harrisburg County. However, in 1839, the name changed to Harris County after John R. Harris.

4. Maricopa County, Arizona

With Phoenix as the county seat in this county, Maricopa County has a total of 4.4 million people. While this sounds like a lot, it stretches out over quite a bit of space, measuring over 9,200 miles. This means that there are only about 480 people per square mile. 1.63 million of the 4.4 million live in Phoenix. 

Over half of the population of Arizona lives in this county, and as of 2022, it’s the fastest-growing county in the United States. Between 2021 and 2022, over 56,000 people moved into the area. In addition to being the most populated county in Arizona, it is full of rich history and is one of the oldest counties in the state.

5. San Diego County, California

California comes back for a second time on this list with San Diego County, California. This county is located in the southernmost part of the state. In fifth place, San Diego County has 3.3 million people. With a land area of 4,207 miles, there are only about 779 people per square mile. 

One unique feature about San Diego County is that it is home to 18 tribal reservations, which makes it the county with the most reservations in any state. There are also 70 miles of coastland within the area. Until 1848, this chunk of the state belonged to Mexico.

6. Orange County, California

Orange County comes in close on the list. It’s number six of the most populated counties in the US with 3.19 million people. Despite this, it’s a heavily populated area based on density when you consider that it only takes up 790 miles. That means that there are about 4,031 people per square mile.

The county seat is Santa Ana. It’s one of the metropolitan areas of Los Angeles, which explains the dense population. In fact, it’s the second most densely-populated county in California, following behind San Francisco County. Most of the population is combined into three cities; Anaheim, Santa Ana, and Irvine.

7. Miami-Dade County, Florida

Miami-Dade County in Florida comes in seventh with a population of around 2.70 million. The county takes up 1,898 miles, which makes the population density around 1,424 people per square mile.

Though Miami, a major city, is part of Miami-Dade County, a lot of the area is made up of natural parks and agricultural land. This lends itself to the lower population level even though Miami and some other parts of the county are heavily populated. The Everglades National Park stretches out to the west, while Biscayne National Park and Aquatic Preserves are to the east.

8. Dallas County, Texas

Dallas, located in Dallas County, Texas, has almost 50 percent of the county’s population with 1.22 million of the 2.6 million people living in the city. The county is small enough that the population density is right about 3,000 people per square mile. Dallas County mostly encompasses the city, though it stretches out a little in every direction. 

It was founded in 1846. There’s not a lot of information about its founding, but it’s thought that it was named Dallas after the 11th Vice President of the United States, George Mifflin Dallas.

9. Kings County, New York

Kings County in New York is the most densely populated county on this list, with a population of just under 2.6 million in an area of less than 71 square miles. This makes the population density roughly 36,579 people per square mile.

The county only covers Brooklyn, so this population density shows a lot of information about how crowded big cities like Brooklyn are. Technically, though, Brooklyn is only a borough of New York and not its own city.

Summary of the Most Populated Counties in the US