Entering the US with ESTA Visa waiver program with a one way flight and a bus ticket to Mexico

@Phoog’s answer is of course correct but I want to add one note.

It’s important when entering with a visitor visa that the USA properly records your departure from the USA. Otherwise, the next time you try to enter you might get flagged as an overstayer.

Unfortunately, the USA complicates this by not having exit controls. If you were leaving by airplane, then the airline company would record your departure to the USA through APIS. But you’re taking the bus.

Bus companies do not (always) report passenger data to the USA. And private cars and taxis of course don’t. So the USA is supposed to get information from the Mexican (and Canadian) passport agencies when you enter those countries. But this doesn’t always happen and sometimes your departure record doesn’t get recorded.

So if you leave via Mexico, I would:

• Keep your bus or car service ticket. Keep a photo of it on your phone.
• Make sure the Mexican border agent properly stamps your passport with a legible date
• A few weeks later, check your USA CBP entry exit (I-94) record online to make sure the USA knows that you have left. If it hasn’t, there are instructions on where to send a copy of your passport information and evidence of your departure (bus ticket, copy of your passport page with the Mexican entry stamp)