Kelly was originally asked to play Nymphadora Tonks in Harry Potter and the Order of the Phoenix before Natalia Tena was cast. She was also cast in the award winning show Boardwalk Empire, which was on the channel HBO.
Her noteworthy performances include Ewan McGregor's underage girlfriend in Trainspotting (with Shirley Henderson and Peter Mullan), the plucky Scottish maid Mary in Robert Altman's ensemble mystery film Gosford Park (serving Maggie Smith's character, and appearing with Stephen Fry, Michael Gambon, Geraldine Somerville, and Sophie Thompson), and the romantic comedy/drama The Girl in the Cafe, directed by David Yates and co-starring Bill Nighy. Her most recent starring role was the voice of Princess Merida in Disney/Pixar's Brave, the cast of which also included Patrick Doyle, Robbie Coltrane, Emma Thompson and Julie Walters.
She re-united with several members of the original cast of Trainspotting, including McGregor and Henderson, for the sequel T2: Trainspotting, also starring Katie Leung.