The airline called Miami-Dade police and, once the plane landed, officers escorted a woman off the plane at Miami International Airport without incident.
A spokesperson for the Miami-Dade Police Department said American Airlines staff dealt “administratively” with the passenger, which involved her being placed on the airline’s no-fly list pending further investigation.
It was unclear if the passenger, a woman in her 40s, was British or American.
She was not arrested, as she did not break local law. In September, President Joe Biden increased the federal penalty for people refusing to mask up on planes to $500.
The flight, which had been in the air for 90 minutes, was cancelled as a result of the incident.
A statement from the American Airlines said: “Flight 38 with service from Miami (MIA) to London (LHR) returned to MIA due to disruptive customer refusing to comply with the federal mask mandate.
“The flight landed at MIA where local law enforcement met the aircraft. We thank our crew for their professionalism and apologize to our customers for the inconvenience.”
Other passengers were unaware of the incident but told they would be returning to Miami and put up in hotels overnight until they could re-book on Thursday.
One passenger, Theresa Green, said American Airlines “looked after” her and her fellow travellers.
She said: “They looked after us. They put us up in nice hotels.”
American Airlines said there were 129 passengers and 14 crew members on the plane.
Pilots performed the U-turn of the Boeing 777 aircraft around about 500 miles into the 4,400-mile transatlantic flight.
Initially, many of them were confused regarding the turnaround.
“We weren’t given any instructions, just told that we had to go back and now we can’t get our luggage,” said one passenger to CBS4.
“They wouldn’t really say anything and I don’t think they told the flight attendants anything because all they would say is there was an extreme incident and they had to turn around,” said another passenger.
In mid-December, American Airlines chief executive Doug Parker said he does not think masks should be required on planes any longer, despite cases of the newly identified Omicron variant spreading across the country.
The Transportation Safety Administration first announced its mask mandate for all passengers last year. It will expire in March.
Airlines reported nearly 6,000 incidents involving unruly passengers last year and 151 in the first two weeks of this year, according to the Federal Aviation Administration.
Most of them involved passengers who refused to wear masks.