The two prizes capped a weeklong surge of Hollywood honors for the British monarchy saga, which is building momentum for the Feb. 27 Academy Awards, where the Facebook drama "The Social Network" previously had looked like the favorite.
Natalie Portman earned the best-actress award at the Screen Actors ceremony for "Black Swan," while "The Fighter" co-stars Christian Bale and Melissa Leo swept the supporting-acting honors, boosting their own prospects come Oscar night.
"The King's Speech" leads Oscar contenders with 12 nominations, among them best picture and actor for Firth, who has been the awards favorite virtually since the film premiered at festivals half a year ago.
"Until today, I would say probably, if ever I felt that I had a trophy which has told me that something's really happening for me, it was my SAG card," said Firth, who plays Queen Elizabeth's dad, George VI, as he takes the throne in the 1930s while struggling to overcome a debilitating stammer.
"Growing up in England, it's not something you expect to see in your wallet, really," Firth continued. "And so it has this glow, and I used to flash it around, hoping it would get me female attention, entry into nightclubs and top-level government departments. It didn't."
Many winners had gushing words for the protection and fellowship their union offers.
"I've been working since I was 11 years old, and SAG has taken care of me," said Portman, who won for her role as a ballerina losing her grip on reality. "They made sure I wasn't working too long and made sure I got an education while I was working."
Bale is a strong favorite for the supporting-actor Oscar as real-life fighter Dicky Eklund, whose career unraveled amid drugs and crime. Eklund briefly joined Bale on stage, the actor telling him he's "a real gentleman."
"I love acting. I love what we do," Bale said. "It's so bloody silly at times, isn't it? It's like playing dress-up, and other times it is so meaningful. I just enjoy that so much - we get to walk in other people's shoes. Life without empathy is no fun at all. "
Leo, who plays the domineering matriarch of a boxing family in "The Fighter," was speechless for a long moment after taking the stage.
"I'm much better when I have my words written for me and somebody's costumes to put on," said Leo, 50, an Oscar nominee two years ago for "Frozen River" who had success earlier in her career on TV's "Homicide: Life on the Street" but has caught a second wind at an age when many actresses find roles scarce. "This has been an extraordinary season for me."
Betty White, who is having her own career resurgence in her 80s, won for TV comedy actress for "Hot in Cleveland."
"I must say this is the biggest surprise I've ever had in this business. There wasn't a prayer. I am so lucky to be ... at 89, to be working ...," White said, pausing as the crowd interrupted her with effusive applause. "You didn't applaud when I turned 40."
Alec Baldwin won his fifth-straight guild award for best actor in a comedy series for "30 Rock."
"I don't know what to say. This is ridiculous. I'm so happy," Baldwin said. "We've had a great year with the show."
"Modern Family" won for overall cast performance in a TV comedy.
Steve Buscemi of "Boardwalk Empire" and Julianna Margulies of "The Good Wife" won as best actors in a TV drama. "Boardwalk Empire," a Prohibition-era gangster series, also won for overall TV drama cast performance.
Buscemi's thanks included a shout out and congratulations to Martin Scorsese, who won a Directors Guild of America Award prize the night before for directing the pilot episode of "Boardwalk Empire." Scorsese was ill and unable to attend.
"Marty, we love you. We hope you feel better, and we love working with you. Please come back," Buscemi said.
Margulies had warm words for her in-laws "for producing truly the most spectacular human being, who I get to call my husband." Before the show began, the guild presented its award for film stunt ensemble to the sci-fi blockbuster "Inception" and the TV stunt prize to the vampire drama "True Blood."
"The Social Network," chronicling the rise of Facebook, had been the early Oscar favorite for best-picture, named the year's top drama by key critics groups and the Golden Globes.
But "The King's Speech" has surged forward in the past week, pulling upset wins at the Directors Guild and Producers Guild awards and leading the Oscar field at last Tuesday's nominations. "The Social Network" had two SAG nominations but came away empty-handed.
Last year's individual winners at the guild awards - Bridges for "Crazy Heart," Sandra Bullock for "The Blind Side," Mo'nique for "Precious" and Christoph Waltz for "Inglourious Basterds" - all went on to win at the Oscars.
The cast prize, considered the guild's equivalent of a best-picture honor, has a spotty record at predicting the top Oscar winner.
The recipient of the guild's cast award has gone on to claim best-picture at the Oscars only seven of 15 years since SAG added that prize. Last year's guild cast recipient, "Inglourious Basterds," lost out to "The Hurt Locker" in the Oscar best-picture race.
The 17th annual SAG Awards, held at the Shrine Auditorium in Los Angeles, included a life-achievement honor for Ernest Borgnine.
"We are a privileged few who have been chosen to work in this field of entertainment," said the 94-year-old Borgnine, whose award was preceded by a tribute including clips from his Oscar-winning performance in 1955's "Marty" through his role in last fall's action comedy "Red." "I hope that we will never let our dedication to our craft fail, that we will always give the best we possibly can to our profession."