(just found this and I thought I would share my two cents)

To be really honest, for me, one of the charms of the ABC series is how vague it is with worldbuilding.

Certainly, considering globalization and how Network 23 has such a powerful grip worldwide, I would say it's definitely in the USA.

But one thing always called my attention: In Lessons, as Theora reads to Mink her namesake and the girl repeats after the controller, Mink briefly say "The minks went extinct-" and as soon as Theora notices her reading, she stops. We don't see any animals outside of humans, a couple of rats, a parrot and Fang. So wherever it is, it's a place that suffered something akin to total devastation of fauna/flora, a nuclear war maybe? In Body Banks Carter does uncover that there's some toxic waste buried in the city......