Dallas Love Field served 5.6 million passengers in 2002. They imply that the number may be 7 million now.
Served by Southwest Airlines, Continental Express and Delta Connection ASA to 17 U.S. cities. Customs service only available on an on-call basis.
BTW, I’m not saying that Love Field is the busiest non-international airport. Just adding a data point.
If Love Field isn’t the winner it’s certainly in the running.
Most flights from Canada are pre-cleared, meaning there US customs and Immigration officers there and they clear the flight to land anywhere. When you land, you land at a domestic terminal and you can obviously take off from anywhere. So flights to and from Canada really don’t count.
My first guess would have been Laguardia, but maybe they do have real international flights, although I have never heard of it.
I would go with what panamajack said - Van Nuys must be your winner in that category. It has no regularly scheduled flights at all, and it is the 7th most busy airport in the world. More aircraft go in and out of there than Heathrow.
I’ll try to get the cites back but I read that Hobby in Houston was busier than Love field. (see dept. of transportation) They have numerous stats. sheets pertaining to US airports. My modem hung up while I was onsite. I’ll get back to it ASAP.
major? 
Sorry xayoz306 I just couldn’t help myself.
Also, aren’t La Guardia’s runways supposedly too short for takeoff of fully fueled large aircraft (intercontinental)?
Depends on the airplane, to some extent, but La Guardia’s runways are listed as 7000 feet in the airport directory. Doesn’t sound terribly long for a jumbo jet to me.
The runway at Leeds Bradford Airport in the UK is listed as 7381 feet, and I know that both 747s and Concorde have been there in the past. http://www.world-airport-codes.com/united-kingdom/leeds/bradford-4041.html
Also, not all international or intercontinental flights are on jumbos. The 767, for instance, is approved for transatlantic routes (with some caveats), and has been used for routes without the traffic to justify a 747.
A-ha…the NY Port Authority limits non-stop La Guardia flights to 1500 miles. Hence nothing transatlantic.
Finally I’ve found some decent data on passenger numbers. Love Field was number 56 in N America in 2002…now to see if there’s any higher. Oh boy, I’m a sad sad person.
BTW, I think we have to count Canadian flights as international, no matter where the customs checks take place. And Van Nuys doesn’t count, because passenger numbers are what we’re considering.
Anybody want to improve on John Wayne Airport, almost 8m passengers and nothing international?
BTW, the largest non-international in the UK seems to be Newquay, with 2 737s to London and a couple of small flights to the Scilly Isles each day. Puts us in perspective, lol.
Van Nuys is an interesting answer, but John Wayne (or whatever beats it, if such an airport exists) is probably the real answer to my question, which was intended to be about passenger travel.
And GorillaMans’ right – Canada, Mexico and Caribbean countries all count as international flights for my question’s sake.