The royal couple, both 31, were guests at the premiere of Natural History Museum Alive 3D, alongside iconic British wildlife presenter Sir David Attenborough.
Kate – in a black lace Alice Temperley gown she wore last year at the premiere of the World War I epic War Horse – and William, in a smart tuxedo, gave some 21st-century glamour to the stunning Victorian surroundings of the museum, which is just a stone's throw from their Kensington Palace home.
The outing was Kate's first official duty in her role as patron of the museum and was also the couple's last public engagement before they settle down for their son Prince George's first Christmas.
It was the second engagement of the day for the pair, who earlier made a private visit to South Africa House to sign the book of condolence in memory of the late Nelson Mandela.
The visit to the museum wrapped up a busy few weeks for the new parents, which saw William sing with Taylor Swift at a charity gala and the couple attend the premiere of Mandela: Long Walk to Freedom.
Inside the museum, the Duchess chatted animatedly with Attenborough about what kind of creatures they could expect to see in the new nature film, which was in 3D.
She then received a set of stills from the film, a gift from the children of the CEO of the production company, Jessica and Francis Geffen, both 3½.
Their father, Anthony Geffen, told reporters, "She is brilliant with children and asked them about Christmas and whether they had seen the film at all – they have seen some clips. It's wonderful that Sir David is bringing his brilliance to a new generation."
William and Kate were shown to their seats in the grand entrance – which houses the museum's most famous resident, a diplodocus dinosaur called Dippy – and Kate giggled as she was handed her 3D glasses but gamely donned them alongside her husband.
Suzanne Plunkett / Reuters / Landov