And she cussed on stage!
The award was announced by acting legend Kirk Douglas, 94, in a speech that had all the nominees on the edge of their seats.
"You pinch me?" she asked of Douglas on the stage, before returning his mumbled compliment with "you're pretty good looking yourself."
"Really, truly wow," she added. "Thank-you."
Then there was a moment when she said a word she shouldn't, which was silenced in the broadcast. Leo was assisted off the stage -- and into Oscars history, owing to her lapse -- with the aid of Douglas's cane.
The veteran actress picked up awards season momentum in January when she also took home a supporting actress award from the Golden Globes.
Leo beat Helena Bonham Carter ("The King's Speech"), Amy Adams ("The Fighter"), Melissa Leo ("The Fighter"), Jacki Weaver ("Animal Kingdom") and newcomer Hailee Steinfeld ("True Grit").
"The Fighter," a critical hit, also won praise for Best Supporting Actor nominee Christian Bale and co-star Mark Wahlberg.