The 50-year-old star capped off her amazing 2011 awards season by triumphing over her The Fighter co-star Amy Adams to claim Oscar gold. She also beat competition from Helena Bonham Carter (The King's Speech), Hailee Steinfeld (True Grit) and Jacki Weaver (Animal Kingdom).
Accepting the honour from 94-year-old acting icon Kirk Douglas, Leo paid tribute to her movie co-stars Adams and Mark Wahlberg, and thanked her family for their support.
But she also turned the air blue, swearing just moments into the live telecast as she marvelled at the packed audience inside the Kodak Theater, where the awards are taking place.
She gasped, "When I watched Kate (Winslet, 2008 Oscar winner) three years ago it looked so f**king easy!"
Anne Hathaway and James Franco opened the 83rd Annual Academy Awards by spoofing Best Picture nominee Inception and attempting to break into 2010 presenter Alec Baldwin's dreams to find the secrets to success as the co-hosts of the biggest prizegiving in Hollywood. Morgan Freeman also featured in the hilarious segment, which saw the young stars appear in clips from other top film nominees The Social Network, The Fighter, True Grit, The King's Speech and Black Swan.
The very first award of the night, for Best Art Direction, went to Robert Stromberg and Karen O'Hara for their work on Alice In Wonderland, while Inception's Wally Pfister landed Best Cinematography.
Toy Story 3 took home Best Animated Feature.