The Downton Abbey actress was utterly enchanting as she swept up the red carpet on Sunday night (1 March) clad in a beautiful Elie Saab Haute Couture gown.
Lily – who plays Lady Rose in Downton - stars as Ella in the new Disney movie which brings the company's beloved animated fairy tale to life on the big screen.
"I'm having so many dream-come-true moments I can't even count them," she told Daily Mail.
"Getting Downton was one of them. I'd watched the show from the start and loved it so I couldn't believe it when I got the job. Now to play a Disney princess is just silly really. It's hard to comprehend. I don't want to be one of those actors who can't admit it's exciting."
Other stars in the movie include Helena Bonham Carter as the Fairy Godmother, Cate Blanchett as the Evil Stepmother and Game of Thrones's Richard Madden as Prince Charming. (Lucky Lily!)
There's also another Downton Abbey favourite in the cast: Sophie McShera! The actress, who plays Daisy in the ITV drama, stars as one of Cinderella's Wicked Stepsisters.
Cinderella, directed by Sir Kenneth Brannagh, arrives in cinemas on 27 March.
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