| County | FIPS Code | County Seat | Established | Population | Area (sq mi) | Area (km2) |
|---|---|---|---|---|---|---|
| Fairfield County | 1 | Bridgeport | 1666 | 916,829 | 626 | 1,621 |
| Hartford County | 3 | Hartford | 1666 | 894,014 | 736 | 1,906 |
| Litchfield County | 5 | Litchfield | 1751 | 189,927 | 920 | 2,383 |
| Middlesex County | 7 | Middletown | 1785 | 165,676 | 369 | 956 |
| New Haven County | 9 | New Haven | 1666 | 862,477 | 606 | 1,570 |
| New London County | 11 | New London | 1666 | 274,055 | 666 | 1,725 |
| Tolland County | 13 | Rockville | 1785 | 152,691 | 410 | 1,062 |
| Windham County | 15 | Willimantic and Putnam | 1726 | 118,428 | 513 | 1,329 |
