The British star won the prize for his role in Tom Hooper's 'The King's Speech' - following on from his Golden Globe win earlier in January - and upon accepting his award, he joked about the calibre of talent in the room at the event, saying he was surprised to have been let in.
He said: "I'd like to thank, looking out at who's here, I'd like to thank security for letting me in the building."
He also paid tribute to his fellow actors in the film, branding three of them "extraordinary".
In addition to the Outstanding Actor prize, the film also won the award for Outstanding Performance by a Cast for the movie about stuttering British monarch King George VI.
Pregnant Natalie Portman also repeated her success from the Golden Globes, winning the Outstanding Actress award for her performance in dark ballet movie 'Black Swan'.
Accepting her trophy, she revealed her parents have been behind her success as they helped to keep her grounded.
She said: "My parents taught me to work the hardest and never be an a*shole. That wasn't acceptable."
'The Fighter' - about Irish boxer Micky Ward - was the other big winner in the movie section of the awards, scooping Outstanding Supporting Actor and Actress prizes for Christian Bale and Melissa Leo respectively.
In the television awards, Al Pacino won the Outstanding Actor nod for his role in 'You Don't Know Jack', while music TV show 'Glee' walked away empty handed despite being up for three potential trophies.
Complete list of winners at the 2011 Screen Actors Guild Awards:
Outstanding Performance by a Cast in a Motion Picture:
'The King's Speech'
Outstanding Performance by a Male Actor in a Leading Role:
Colin Firth, 'The King's Speech'
Outstanding Performance by a Female Actor in a Leading Role:
Natalie Portman, 'Black Swan'
Outstanding Performance by a Male Actor in a Supporting Role:
Christian Bale, 'The Fighter'
Outstanding Performance by a Female Actor in a Supporting Role:
Melissa Leo, 'The Fighter'
Outstanding Performance by an Ensemble in a Drama Series:
Outstanding Performance by an Ensemble in a Comedy Series:
Outstanding Performance by a Male Actor in a Drama Series:
Steve Buscemi, 'Boardwalk Empire'
Outstanding Performance by a Female Actor in a Drama Series:
Julianna Margulies, 'The Good Wife'
Outstanding Performance by a Male Actor in a Comedy Series:
Alec Baldwin, '30 Rock'
Outstanding Performance by a Female Actor in a Comedy Series:
Betty White, 'Hot in Cleveland'
Outstanding Performance by a Male Actor in a Television Movie or Miniseries:
Al Pacino, 'You Don't Know Jack'
Outstanding Performance by a Female Actor in a Television Movie or Miniseries:
Claire Danes, 'Temple Grandin'
Screen Actors Guild Awards 47th Annual Life Achievement Award
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