Leo earned the honor for her portrayal of a Lowell, Mass., woman who stopped at nothing while managing her sons' boxing careers.
Bale played a pugilist whose addiction to drugs cost him his own career, as well as threatened that of his nice-guy brother, played by Mark Wahlberg.
"Mine? For me?" Leo asked presenter Kirk Douglas as she took the stage to claim her Oscar statuette. When Douglas, 94, told her she is more beautiful in real life than she was in the film, Leo flirted back: "You're pretty good-looking yourself. What are you doing later on?"
Turning back to the crowd, she exclaimed: "Oh my God! Oh, wow! I know there's been a lot of people who said a lot of real, real nice things to me for several months now, but I'm just shaking in my boots here."
"Bloody hell! What a room full of talented and inspirational people. What the hell am I doing in the midst of you? It's such an honor," Bale wondered as he accepted his own Oscar.
"The Lost Thing" was named Best Animated Short and "Toy Story 3" declared Best Animated Feature.
Aaron Sorkin picked up the trophy for Best Adapted Screenplay for "The Social Network," which also earned Trent Reznor and Atticus Ross the prize for Best Original Score. "Network" won the Film Editing award, as well.
David Seidler took home the statuette for Best Original Screenplay for "The King's Speech," "Inside Job" landed the Best Documentary Feature award and Denmark's "In a Better World" garnered the gong for Best Foreign Language Film.
"Alice in Wonderland" won the Oscars for Best Art Direction and Best Costume Design. "Inception" picked up the prizes for Best Cinematography, Best Sound Editing, Visual Effects and Best Sound Mixing. "The Wolfman" was awarded the Oscar for Best Makeup.