The couple, both 31, were the guests of honor at the U.K. premiere of Mandela: Long Walk to Freedom.
Kate, in a favorite white Roland Mouret gown that she previously wore to a private dinner in May 2012, and William, in his traditional tuxedo, lit up the chilly West End evening.
The outing comes after a heady 10 days for the two, during which William sung alongside Taylor Swift at a charity gala and Kate led the applause for young athletes at an annual celebration held by SportsAid, one of her patronages.
Thursday night's royal film premiere saw them mingling with actors Idris Elba, who plays the revered former South African president, who died Wednesday, and Naomie Harris, who stars as his ex-wife Winnie.
Asked about having William and Kate along as special guests, Elba told reporters, "Tonight is extra special because this is my country, and when the royals come out to watch a film, it's a big deal. I feel very proud that it's my film, really proud."
Elba also talked about his transformation into the iconic figure, saying that he "worked really hard to pull in all the elements, from the sound of [Mandela's] voice to the way he walks. (He's certainly gotten some major approval: One of Mandela's daughters, Zindzi, was at the premiere and said that her father jokes that he thought Elba was him.)
For her part, Harris, wearing one of Kate's favorite labels, Alexander McQueen, told PEOPLE that it was "fantastic for us" to have the royals at the premiere. "It's such a huge honor. We feel really proud to have them here."
Harris has actually met William several times at polo games, she revealed. "He's lovely. He's very down to earth and charming."
Anticipating meeting Kate, Harris added, "I hear she's just as down to earth and super charming as well."