I'm not sure it fully qualifies as an appropriate answer, since it was de facto law before the US existed ... and I'm not entirely sure what the statutes were between when the US declared independence and when it had its first constitution ... but the Spanish dollar was legal tender in the US, coexisting with the US dollar, until 1857. That makes it an externality that turned into US law, arguably owing to pragmatism but also because the USD was of very similar weight.
A more recent example might simply be Roe vs Wade. No law was passed, but because abortion was becoming legal elsewhere, because of on the ground activists, and because of activist lawyers who filed lawsuits, SCOTUS ended up sensing the winds of change, took on a case (or the latter two the other way around, but it doesn't matter), and basically legalized abortion without getting Congress involved. It's probably stretching the truth to say that SCOTUS changed the law only because of foreign influence, but I can't imagine a scenario where the latter didn't play a role as Justices wondered what was happening elsewhere. Similarly for gay marriage, or for the ongoing cases that are trying to make their way to the Supreme Court to legalize marijuana or ban child corporal punishment.
The situation the other way around, with US law become another country's law, happens basically every day: because of 1st amendment free speech protection, just about anyone anywhere can publish to their heart's content with a large degree of impunity. It might not be accessible to the locals in their country, and whoever does publish might end up prosecuted in their country if their government finds out who they are ... but they can publish it if they've the technical skills to work around whatever firewall they're behind, and their fellow citizens can read what they publish if they have that skillset too. It's not technically a law change, of course, but it's prompting other countries to look into whether the law or how it's enforced should be changed. (I'd also point out in passing that it works the other way around for speech restrictions. Hardly any news outlet ill will publish prophet depictions and caricatures.)
A more concrete and related example are the provisions in US law to protect online platforms like Facebook or Twitter. (Section 230 if memory serves me well, but I'd need to double check.) It basically led to the EU Copyright Act from a few weeks ago, which in one sense provides for the opposite by making platforms like Facebook liable for copyright infringements.