The 58-year-old actor, who was a front-runner for the recognition at the 90th Academy Awards, scored her maiden trophy today which was her first nomination in the category.
Names such as Mary J Blige (Mudbound), Lesley Manville (Phantom Thread), Laurie Metcalfe (Lady Bird) and Octavia Spencer (The Shape of Water) were also vying for the title.
Janney was presented the trophy by last year's Best Supporting Actor winner Mahershala Ali.
"Ok, nothing further from the truth," she said as she proceeded to thank her fellow nominees, her movie's team and her parents.
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Janney had already won a Golden Globe, a SAG-AFTRA, a CCA and a Bafta for the role of infamous American figure skater, Tonya Hardings mother LaVona Fay Golden, who raised her daughter with a feeling that she was a good-for-nothing.
The sports drama is a biopic on real-life Olympic figure skater Tonya Harding based on the true events, which saw Tonya's now-ex-husband Jeff Gilooly plot with her self-appointed bodyguard, Shawn Eckhardt, to take out her rival Nancy Kerrigan to pave the way for her spot on the US team.
Janney's LaVona, who is a diner waitress by profession, is a foul-mouthed smoker who can push a young Tonya off her chair and even throw a knife at her adult version in Margot Robbie to prove her point to her daughter.
The actor has earlier described her part as a "distinct, unique mother of a character", a role for which she has repeatedly thanked her Neighborhood Playhouse School of the Theatre classmate and friend Steven Rogers, the screenwriter of the sports drama.
A seven-time Primetime Emmy Award winner for her television work, the actor's breakout role was as White House Press Secretary-turned-Chief of Staff C J Cregg on the NBC drama "The West Wing" (19992006). She presently plays a recovering alcoholic mother, Bonnie Plunkett on the CBS sitcom "Mom".
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