Halfway into her flight from Atlanta to Los Angeles Monday night, Brooke Tansley heard the pilot’s voice pipe up through the public address system, apologizing for the interruption.

“You’re going to be very excited to hear this news,” she recalled him saying. The Transportation Security Administration was no longer requiring masks on planes, and passengers were free to to take them off if they’d like. The announcement was met with a smattering of claps and some jubilant cheers, and about a third of the people aboard immediately peeled off their masks, Ms. Tansley said.

For some travelers, Monday evening’s announcement from the T.S.A. that it would stop enforcing a mask mandate after a federal judge in Florida struck down the requirement came as they were already on their way — in airport terminals, on the tarmac or even in the air.

Many passengers greeted the news with applause and cheers, as seen in videos on social media. One took a celebratory selfie, with most fellow passengers in wide, maskless grins. A pilot told those aboard his flight: “Congratulations.”

Not all flights appeared to have immediately done away with the mask requirement. At least one person wrote on Twitter that a flight attendant told him masks had to stay on.

For her part, Ms. Tansley felt a jolt of fear and alarm. She was with her two children, a 4-year-old and an 8-month-old, both too young to be vaccinated, the baby too young for a mask. She was on her way to a work meeting involving a colleague with a rare autoimmune disease, and her family had undergone P.C.R. tests because they were worried about potentially infecting him.

“I was scared — all I could do was hope it’s going to be OK,” she said. “There wasn’t any other option.”

Ms. Tansley said her family hadn’t been on a flight since Christmas 2019 out of concern about the virus. She has asthma, and said she wasn’t sure whether she would go ahead with her work meetings, or what her family would do about their return flight home on April 25.

“It’s not that the mask mandate has changed that upset me, it’s that we boarded the plane under one set of rules, and made a decision as a family and as a work group,” said Ms. Tansley, a television producer and former Broadway performer. “The decision was made for us midflight.”

Scott Hechinger, a lawyer, was waiting for his delayed flight to New York at a crowded terminal in West Palm Beach, Fla., when the announcement came that the mandate had been lifted and passengers were free to board their flights without masks.

The terminal erupted in a loud, sustained cheer, and about half the travelers took off their masks, he said.

“For this announcement to happen literally minutes before we got in the plane made me feel very uncomfortable,” Mr. Hechinger wrote in a text message. On his flight, about 75 percent were unmasked, he said.

He said he worried about his wife and 6-year-old son, who were in the air on a separate flight en route to Los Angeles.

“It hit me that my wife and young son would also likely get this announcement midflight and be more exposed than usual,” he wrote. “I’m upset, uncomfortable, and frustrated.”