The statuesque 58-year-old Janney, the overwhelming favourite in the category, bested another actress playing a stern mother, Laurie Metcalf in Lady Bird, as well as Octavia Spencer for The Shape of Water.
"My fellow nominees, you represent everything that is good and right and human about this profession," she said.
Janney had already bagged the Screen Actors Guild Award, a Bafta and her first trophy in January at the Golden Globes, where she said the movie told "a story about class in America, the disenfranchised."
Harding, who became notorious after her ex-husband and bodyguard hired someone to attack her skating rival Nancy Kerrigan, has appeared with the cast during the awards season.
Authorities later found she was aware of the plan, and she was banned from the sport for life.
Janney has said the film is about "a woman who was not embraced for her individuality, the truth and the perception of truth in the media and the truths we all tell ourselves when we wake up in bed every morning and go out and live our lives."
Though she has appeared in some major box office hits, Janney is best known for her portrayal of White House press secretary C.J. Cregg in the popular NBC drama The West Wing, winning four Emmys and two SAG awards for the role.
In 2014, she became the third performer to win Emmys for different roles in the same year -- supporting comedy actress in the CBS sitcom Mom and outstanding guest actress in Showtime’s Masters of Sex.
She won her seventh Emmy in 2015 -- again for Mom -- to tie with Edward Asner and Mary Tyler Moore for second most by a performer, behind Cloris Leachman and Julia Louis-Dreyfus on eight.
The actress described The West Wing as an "extraordinary experience" that had connected her and her co-stars forever as she accepted a star on Hollywood’s Walk of Fame in 2016.
"I didn’t even know it was going to be a series. None of us thought it would go on -- not because it wasn’t brilliant but because it was about politics and who wants to watch a show about politics? Apparently a lot of people," she said.
Raised in Dayton, Ohio, Janney’s most recent movies include Tim Burton’s dark fantasy Miss Peregrine’s Home for Peculiar Children and mystery thriller The Girl on the Train, which made almost US$400 million between them in 2016.
She also supplied the voice of the pink starfish Peach in the Pixar animated adventure Finding Dory, that year’s largest-grossing movie, and appeared in two of 2015’s biggest hits, the live-action caper Spy and animated comedy Minions.
Among her other film credits are award-winners The Help, Juno, The Hours and American Beauty.
Richard Schiff, who played White House communications chief Toby Ziegler in The West Wing, recently described Janney as "the most amazing talent to grace large screen and small." Source from Vietnamnews.