Other film honors went to Natalie Portman for her starring role as a ballerina in "Black Swan" and to Melissa Leo and Christian Bale for their supporting work in "The Fighter."
"It shouldn't be called the SAG award; it should be called the uplifting award," Geoffrey Rush quipped as he accepted the ensemble award on behalf of the cast of "The King's Speech."
It's the guild's equivalent of best film of the year and positions the movie about King George VI's attempts to stop stuttering as the nominee to beat at the Academy Awards next month. The win came moments after its star, Firth, won for his performance as the king.
Though "The Social Network" had the most momentum at the beginning of the awards season by winning the majority of critics honors as well as the Golden Globe for best dramatic film, "The King's Speech" has been gaining speed in recent weeks: It won the Producers Guild Award last weekend and director Tom Hooper won the Directors Guild Award Saturday night. The film leads in Oscar nominations, with 12 nods.
It didn't make a clean sweep Sunday, however. Both Rush and Helena Bonham Carter were nominated for their supporting roles but lost to Bale and Leo in "The Fighter."
Leo's win for playing the mother of two boxers was her first at the SAGs, but two weeks ago she also won the Golden Globe for supporting actress for the role and will be competing again at the Oscars.
"This has been an extraordinary season for me," Leo said.
Like Firth, Portman's win Sunday solidfies her front-runner status heading into the Oscars, coming on the heels of the Golden Globe for her performance as a prima ballerina decending into madness.
"I've been working since I was 11 ... I am so grateful to have this union protecting me every day," said Portman, who is expecting her first child after meeting her fiance on the set of "Black Swan."
On the television side, Al Pacino and Claire Danes won SAGs for performances that previously had earned them Emmys and Golden Globes. Pacino won for portraying the controversial Dr. Jack Kevorkian in HBO's "You Don't Know Jack," while Danes was honored for her performance in HBO's "Temple Grandin."
Betty White, who won SAG's lifetime achievement award last year, was back again Sunday, winning the group's award as lead actress in a TV comedy for her role in the cable series "Hot in Cleveland."
"This is the biggest surprise I've ever had in this business," the 89-year-old actress said.
"Hot in Cleveland" also was nominated for the ensemble comedy award but lost in that category to ABC's "Modern Family."
For the fifth year in a row, Alec Baldwin won as lead actor in a comedy for "30 Rock."
In dramatic TV, HBO's first-year show "Boardwalk Empire" was the big winner, grabbing honors for the ensemble cast and for Steve Buscemi as lead actor. He was honored for his role as a 1920 gangster kingpin, the first time he's won a SAG award. Two weeks ago, he won the Golden Globe for his performance in the show.
Julianna Margulies was the other drama winner, landing the trophy for outstanding lead actress her starring role as a lawyer in CBS' "The Good Wife." She also won last year.
The SAG awards were held at the Shrine Exhibition Hall and telecast live simultaneously on TNT and TBS.
Ernest Borgnine, who won the Academy Award for lead actor for 1955's "Marty" and who just celebrated his 94th birthday, was this year's SAG recipient of the Lifetime Achievement Award. Borgnine, seen most recently in "Red," also starred in such films as "From Here to Eternity," "The Poseidon Adventure," "The Wild Bunch" and "The Dirty Dozen," and in the TV series "McHale's Navy."
"There are millions of those in the world who would love to be in our shoes," Borgnine told his colleagues. "We are a privileged few who have been chosen to work in this field of entertainment.... I hope we will never let our dedication to our craft fail. Thank you so very much."