Kate, 32, was helping the Queen host a reception for British actors and actresses at Buckingham Palace.
A day after Prince William presented Helen Mirren with a fellowship at the BAFTAs, Kate attended Monday's event, which marked the Queen's 60 years as patron of the Royal Academy of Dramatic Art and the contribution that the dramatic arts make to national life.
The evening was given extra significance by the presence of Dame Helen Mirren, who famously played Elizabeth II in The Queen, and Helena Bonham Carter, who played her mother in The King's Speech.
And Kate giggled as she and Mirren discussed William calling the actress "granny" the night before.
Also on the guest list were international stars such as Uma Thurman, Joely Richardson and Joan Collins, who was wearing a glittering gold lame coat. John Hurt, Michael Sheen and Oscar-nominated director Steve McQueen joined British favorites like comedian Lenny Henry.
At the start of the evening as she greeted actors inside the White Drawing Room, the Queen enjoyed a joke from comedian Henry, who quipped about Mirren's performance as the Queen.
Pointing to the 87-year-old monarch and smiling with McQueen, he said, "This is Helen Mirren. She's brilliant."
It was the third engagement for Kate in a week, following her glittering night at the National Portrait Gallery and then a visit to a facility run by one of the charities she supports, the Art Room.
David Crump / Getty