Houston is entirely inside Harris County
Every one of the 178 ZIP codes USPS lists with Houston as the primary place name sits inside Harris County, Texas (see the full Houston ZIP code list), with no cross-county split at the ZIP level the way some major US cities have. That distinction only holds at the ZIP-code level, though. Houston's actual city limits are wider: annexation over the decades pulled small slices of southwest and northeast Houston into Fort Bend and Montgomery counties, so the city itself crosses county lines even though none of its ZIP codes do. Harris County was founded on December 22, 1836, and organized the following March; Houston was named the county seat that same December, and has held the role ever since. The county takes its name from John Richardson Harris, who founded the town of Harrisburg on Buffalo Bayou a decade earlier, in 1826. Virginia takes the opposite extreme from Houston's straightforward single-county answer: Richmond isn't part of any county at all, one of Virginia's independent cities, a legal status that has no equivalent in Texas or almost anywhere else in the country.
Harris County is closing in on the second-largest spot in the US
Harris County's population of roughly 5.05 million currently ranks third nationally, behind Los Angeles County, California and Cook County, Illinois (Chicago). Harris County added about 106,000 residents in a single 12-month period (2023 to 2024), more than any other county in the country, and it has kept growing faster than Cook County since, even though Cook itself returned to growth after a pandemic-era dip rather than continuing to shrink. Harris still trailed Cook by roughly 150,000 people as of the Census Bureau's most recent estimate (July 2025); whether that gap closes further is not yet a settled trend.