Entry quarantine for entering the United States

Entry screening only applies to travellers arriving from certain high-risk countries:

Currently, travel restrictions and entry screening apply only to travelers arriving from some countries or regions with widespread ongoing spread of COVID-19. [Note: U.S. policies are subject to change as the COVID-19 pandemic evolves.]

(source)

So if you are coming from a high-risk country and have COVID-19 symptoms, you may be put into mandatory quarantine.

Other than that, the US still advises self-quarantine for 14-days after arrival but this is not enforced:

You may be screened when you arrive in the United States. After your arrival, take the following steps to protect yourself and others:

1. Stay at home and avoid contact with others. Do not go to work or school for 14 days.

2. Monitor your health for 14 days. Take your temperature with a thermometer two times a day and monitor for fever. Also watch for cough or trouble breathing.

3. Keep your distance from others (at least 6 feet or 2 meters). This is referred to as “social distancing.”

Update (2020-06-24): NY, NJ and CT require travelers from states with high coronavirus rates to quarantine for two weeks (mirror).

Note that some US states are considering having their own quarantine policy, e.g. https://www.today.com/video/ny-gov-andrew-cuomo-we-re-seriously-considering-a-quarantine-for-visitors-from-out-of-state-85804101542 (mirror) (2020-06-23):

Andrew Cuomo, governor of New York, tells TODAY that “we went from one of the highest infection rates in the Unites States to one of the lowest,” proving New York state’s “science-based reopening” was the correct approach. He says “we’re seriously considering a quarantine” for out-of-state visitors so they “don’t increase our infection rate.” He also comments on the debate over monuments and on what he calls the “very heavy-handed” dismissal of Geoffrey Berman, former U.S. attorney for the Southern District of New York.