According to a source who attended, the film's producer made the announcement to the crowd with Idris Elba by his side.
"Elba was in tears. Everyone was stunned, it was quite eerie. It felt surreal. It really was extraordinarily eerie," the source told us.
The producer also "asked for a minute of silence, which was observed."
The Duke and Duchess had attended the 2013 Royal Film Performance in London, alongside the cast of the much-talked about film.
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."
According to the same premiere goers, Kate appeared somber as she stood by her husband's side and did not make any comments.
Elba, Naomie Harris and Mandela's daughter Zindzi Mandela also joined the royals in the annual event.
The black-tie affair took a heartbreaking turn when word spread that the former President of South Africa and Novel Peace Prize Laureate had taken his last breath at age 95 before the movie had ended.
Elba and Harris recently opened up on Nightline about what it was like to play the political leader and his wife.
"We were given the task to go ahead and make that as real as possible," he said. "I felt the responsibility to be truthful."
But it was Zindzi and her family's approval that helped Elba fully become comfortable with the performance he had given onscreen.
"The film's great and there's all sorts of talk of awards and things, but when you have Mr. Mandela, the great himself, looking at an image of me and saying, 'Is that me?'—that's it. I've won," he said.
"I've won every possible award you can give, because it's coming from the heart of the man who actually lived it."