They had smiled and posed for photographs on the red carpet and then enjoyed an animated conversation with the Duke and Duchess of Cambridge.
And if Zindzi and Zenani Mandela had any idea that their father's long life was drawing to a close, they did not betray it for a minute.
Then, in a single instance, everything changed. As they were about to watch the London premiere of his incredible story, the sisters were apparently told that the 95-year-old had died peacefully at home after months of illness.
Read more after the cut here...
Shocked: Prince William pays tribute to Nelson Mandela after learning he had died shortly before the London premiere of a new film documenting his life
It was an extraordinary premiere. Few in the Odeon auditorium in Leicester Square knew what had happened and settled down to enjoy an evening celebrating one of the most iconic figures of the 20th century.
In fact, i know how i would have reacted to such negative news...
The Duke and Duchess of Cambridge were informed of his death discreetly by an aide shortly before the end of the movie.
The Queen today expressed her shock at the timing of Mr Mandela's death, saying: 'It was extraordinary because William and Catherine were at the film.
'They were clapping like mad and somebody came on and said, "can you just listen please, we have just heard he has died". It is amazing isn’t it. During the first night of his film.'
It was only after the credits had rolled that the film's producer Anant Singh got on the stage to break the news to the rest of the audience. There were screams and gasps of shock while some burst into tears. A two minutes' silence was held.
Eleanor Simmonds, 54, from Croydon, who was at the premiere to support a Mandela charity, said: 'There was an audible sharp intake of breath. It was shocking news to hear at the end of the film. Everyone was shocked,' she said. 'I was really, really sad news because it was a wonderful evening.'
William and Kate looked ashen-faced as they walked down the stairs into the foyer. Speaking as he left the Odeon cinema, the Duke said: 'I just wanted to say it's extremely sad and tragic news. We were just reminded what an extraordinary and inspiring man Nelson Mandela was.
'My thoughts and prayers are with him and his family. It's very sad.'