He accepted the award moments ago at the Kodak Theatre in Los Angeles from 2010 Best Actress winner Sandra Bullock.
"I have a feeling my career's just peaked," he said from the podium, to laughter.
Firth continued on to thank his fellow cast-members (and designer Tom Ford!), although his halting speech at times more closely resembled his nervous character than his suave self.
The British actor, who was the favorite, beat Jesse Eisenberg ("The Social Network"), Oscars host James Franco ("127 Hours"), Jeff Bridges ("True Grit") and Javier Bardem ("Biutiful").
His co-stars Geoffrey Rush and Helena Bonham Carter were also nominated in supporting categories for their roles in "The King's Speech." To see who else went home with Oscar, check the list of winners at Moviefone.