The 33-year-old, who has been nominated a total of six times, managed to hold back the tears as she accepted the gong.
'I'd be lying if I hadn't made a version of this speech before,' she told the audience.
'I think I was probably eight years old and staring into the bathroom mirror.'
British flick Slumdog Millionaire received the coveted Best Picture prize.
Its haul of 8 Oscars included Best Director for Danny Boyle, 52, and Best Adapted Screenplay and Cinematography.
Sean Penn, 48, won Best Actor for political biopic Milk, while the late Heath Ledger got Best Supporting Actor for The Dark Knight.
Penelope Cruz, 34, claimed Best Supporting Actress for Vicky Christina Barcelona.
Brad Pitt, 45, Angelina Jolie, 33, and Mickey Rourke, 56, were just some of the stars who attended the lavish ceremony at Hollywoods Kodak Theatre.
Hugh Jackman, 40, hosted the event.
Watch Kate's speech here, just press play:
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