U.S. Population – County Population
| Rank | County | 2010 Census | 2000 Census | Growth (%) |
| 1 | Los Angeles County, CA | 9,880,569 | 9,519,338 | 3.8 |
| 2 | Cook County, IL | 5,310,767 | 5,376,741 | -1.2 |
| 3 | Harris County, TX | 4,147,218 | 3,400,578 | 22.0 |
| 4 | Maricopa County, AZ | 4,063,802 | 3,072,149 | 32.3 |
| 5 | Orange County, CA | 3,058,905 | 2,846,289 | 7.5 |
| 6 | San Diego County, CA | 3,076,745 | 2,813,833 | 9.3 |
| 7 | Kings County, NY | 2,584,652 | 2,465,326 | 4.8 |
| 8 | Dallas County, TX | 2,493,102 | 2,218,899 | 12.4 |
| 9 | Miami-Dade County, FL | 2,534,220 | 2,253,362 | 12.5 |
| 10 | Queens County, NY | 2,319,777 | 2,229,379 | 4.1 |
| 11 | Riverside County, CA | 2,176,129 | 1,545,387 | 40.8 |
| 12 | San Bernardino County, CA | 2,040,080 | 1,709,434 | 19.3 |
| 13 | Wayne County, MI | 1,904,853 | 2,061,162 | -7.6 |
| 14 | King County, WA | 1,948,445 | 1,737,034 | 12.2 |
| 15 | Clark County, NV | 1,911,769 | 1,375,765 | 39.0 |
| 16 | Santa Clara County, CA | 1,807,547 | 1,682,585 | 7.4 |
| 17 | Broward County, FL | 1,788,118 | 1,623,018 | 10.2 |
| 18 | Tarrant County, TX | 1,826,770 | 1,446,219 | 26.3 |
| 19 | New York County, NY | 1,631,306 | 1,537,195 | 6.1 |
| 20 | Bexar County, TX | 1,681,428 | 1,392,931 | 20.7 |
| 21 | Suffolk County, NY | 1,521,211 | 1,419,369 | 7.2 |
| 22 | Middlesex County, MA | 1,516,942 | 1,465,396 | 3.5 |
| 23 | Alameda County, CA | 1,504,411 | 1,443,741 | 4.2 |
| 24 | Philadelphia County, PA | 1,558,613 | 1,517,550 | 2.7 |
| 25 | Sacramento County, CA | 1,412,333 | 1,223,499 | 15.4 |
| 26 | Bronx County, NY | 1,402,250 | 1,332,650 | 5.2 |
| 27 | Nassau County, NY | 1,364,745 | 1,334,544 | 2.3 |
| 28 | Cuyahoga County, OH | 1,269,455 | 1,393,978 | -8.9 |
| 29 | Palm Beach County, FL | 1,290,971 | 1,131,184 | 14.1 |
| 30 | Allegheny County, PA | 1,220,510 | 1,281,666 | -4.8 |
| 31 | Oakland County, MI | 1,207,592 | 1,194,156 | 1.1 |
| 32 | Hillsborough County, FL | 1,212,971 | 998,948 | 21.4 |
| 33 | Hennepin County, MN | 1,166,986 | 1,116,200 | 4.5 |
| 34 | Franklin County, OH | 1,159,947 | 1,068,978 | 8.5 |
| 35 | Orange County, FL | 1,100,043 | 896,344 | 22.7 |
| 36 | Contra Costa County, CA | 1,055,952 | 948,816 | 11.3 |
| 37 | Salt Lake County, UT | 1,052,900 | 898,387 | 17.2 |
| 38 | Fairfax County, VA | 1,055,577 | 969,749 | 8.9 |
| 39 | Fulton County, GA | 1,054,582 | 816,006 | 29.2 |
| 40 | Pima County, AZ | 1,027,226 | 843,746 | 21.7 |
| 41 | Travis County, TX | 1,054,052 | 812,280 | 29.8 |
| 42 | St. Louis County, MO | 991,974 | 1,016,315 | -2.4 |
| 43 | Westchester County, NY | 961,499 | 923,459 | 4.1 |
| 44 | Milwaukee County, WI | 965,303 | 940,164 | 2.7 |
| 45 | Montgomery County, MD | 988,521 | 873,341 | 13.2 |
| 46 | DuPage County, IL | 941,097 | 904,161 | 4.1 |
| 47 | Pinellas County, FL | 909,978 | 921,482 | -1.2 |
| 48 | Erie County, NY | 906,245 | 950,265 | -4.6 |
| 49 | Fresno County, CA | 924,691 | 799,407 | 15.7 |
| 50 | Shelby County, TN | 925,852 | 897,472 | 3.2 |
| 51 | Honolulu County, HI | 909,151 | 876,156 | 3.8 |
| 52 | Fairfield County, CT | 908,263 | 882,567 | 2.9 |
| 53 | Bergen County, NJ | 900,766 | 884,118 | 1.9 |
| 54 | Mecklenburg County, NC | 928,585 | 695,454 | 33.5 |
| 55 | Marion County, IN | 897,380 | 860,454 | 4.3 |
| 56 | Hartford County, CT | 881,536 | 857,183 | 2.8 |
| 57 | Wake County, NC | 920,122 | 627,846 | 46.6 |
| 58 | Hamilton County, OH | 855,340 | 845,303 | 1.2 |
| 59 | Duval County, FL | 860,576 | 778,879 | 10.5 |
| 60 | New Haven County, CT | 850,587 | 824,008 | 3.2 |
| 61 | Macomb County, MI | 831,157 | 788,149 | 5.5 |
| 62 | Prince George's County, MD | 839,212 | 801,515 | 4.7 |
| 63 | San Francisco County, CA | 816,159 | 776,733 | 5.1 |
| 64 | Kern County, CA | 814,852 | 661,645 | 23.2 |
| 65 | Ventura County, CA | 810,359 | 753,197 | 7.6 |
| 66 | Gwinnett County, GA | 820,869 | 588,448 | 39.5 |
| 67 | Middlesex County, NJ | 794,857 | 750,162 | 6.0 |
| 68 | Pierce County, WA | 792,919 | 700,820 | 13.1 |
| 69 | Baltimore County, MD | 791,637 | 754,292 | 5.0 |
| 70 | Worcester County, MA | 808,309 | 750,963 | 7.6 |
| 71 | Montgomery County, PA | 789,862 | 750,097 | 5.3 |
| 72 | Essex County, NJ | 769,156 | 793,633 | -3.1 |
| 73 | Collin County, TX | 815,101 | 491,675 | 65.8 |
| 74 | El Paso County, TX | 762,563 | 679,622 | 12.2 |
| 75 | DeKalb County, GA | 752,361 | 665,865 | 13.0 |
| 76 | Essex County, MA | 749,560 | 723,419 | 3.6 |
| 77 | Monroe County, NY | 733,719 | 735,343 | -0.2 |
| 78 | Suffolk County, MA | 759,925 | 689,807 | 10.2 |
| 79 | Hidalgo County, TX | 761,223 | 569,463 | 33.7 |
| 80 | Multnomah County, OR | 738,351 | 660,486 | 11.8 |
| 81 | Jefferson County, KY | 726,844 | 693,604 | 4.8 |
| 82 | San Mateo County, CA | 724,877 | 707,161 | 2.5 |
| 83 | Lake County, IL | 714,252 | 644,356 | 10.8 |
| 84 | Oklahoma County, OK | 727,010 | 660,448 | 10.1 |
| 85 | Cobb County, GA | 719,617 | 607,751 | 18.4 |
| 86 | Snohomish County, WA | 709,534 | 606,024 | 17.1 |
| 87 | Will County, IL | 688,081 | 502,266 | 37.0 |
| 88 | San Joaquin County, CA | 683,494 | 563,598 | 21.3 |
| 89 | Jackson County, MO | 710,455 | 654,880 | 8.5 |
| 90 | Norfolk County, MA | 672,873 | 650,308 | 3.5 |
| 91 | Jefferson County, AL | 665,494 | 662,047 | 0.5 |
| 92 | Monmouth County, NJ | 645,583 | 615,301 | 4.9 |
| 93 | Baltimore city, MD | 634,725 | 651,154 | -2.5 |
| 94 | Denton County, TX | 675,377 | 432,976 | 56.0 |
| 95 | Bernalillo County, NM | 648,641 | 556,678 | 16.5 |
| 96 | Providence County, RI | 630,620 | 621,602 | 1.5 |
| 97 | Davidson County, TN | 641,870 | 569,891 | 12.6 |
| 98 | Bucks County, PA | 626,280 | 597,635 | 4.8 |
| 99 | Denver County, CO | 626,876 | 554,636 | 13.0 |
| 100 | El Paso County, CO | 617,908 | 516,929 | 19.5 |
Source: U.S. Census Bureau, Population Division
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