The royal couple signed the condolence book for the former South African president, who passed away last week.
Both Will and Kate were dressed in conservative clothing as they met with officials.
Kate opted for a polka-dot skirt suit—complete with a perfect blowout, naturally.
Meanwhile, Prince William wore a perfectly tailored navy suit and burgundy tie to the meeting.
Prince George's parents first learned of Mandela's passing while attending the royal screening of Mandela: Long Walk to Freedom.
As they stepped out of the theater, Prince William told reporters that Mandela was an "extraordinary and inspiring" man and that his death was "extremely sad and tragic news."
He added, "My thoughts and prayers are with him and his family right now."
The Duke and Duchess of Cambridge have quite a busy day ahead of them following their South Africa House visit.
The two are set to attend a screening of Natural History Museum Alive in 3-D later this evening.
Kate is a patron of the Natural History Museum in South Kensington and will be one of the first people to view the documentary.
According to British broadcaster and naturalist David Attenborough, the film will "take the viewer on a journey through one of my favorite museums and look at it—and its inhabitants—in ways never seen before.