William, who was celebrating his 42nd birthday that day, and the children later met Swift and her boyfriend, NFL star Travis Kelce backstage.
The official @KensingtonRoyal account posted a photo of Swift taking a selfie with Prince William, Princess Charlotte, and Prince George.
"Thank you @taylorswift13 for a great evening!" wrote the account.
Swift also posted a photo of herself and Kelce with the Royal trio to her official X account. She captioned it: "Happy Bday M8! London shows are off to a splendid start."
Other celebrity fans in attendance included Bridgerton star Nicola Coughlan, Cara Delevingne, Salma Hayek, and Leslie Mann, according to photos shared by Queer Eye's Jonathan Van Ness, who was also at the concert.
Swift opened the show by telling the crowd that there were 88,446 people in the stadium, according to PA Media.
She thanked the crowd for being there, "especially on a Friday night in the summer on a beautiful day in London," and reminisced about gigs she performed in the city earlier in her career.
Swift is set to perform eight shows in London - more than any other city in the world.
According to the Mayor of London, nearly 700,000 people are expected to attend her London shows, which are predicted to boost the capital's economy by 300 million ($379.5 million USD).