Janney, 58, was honoured for her role as the mother of infamous figure skater Tonya Harding, LaVona Fay Golden, in the biopic, both led and produced by Margot Robbie.
She thanked Harding, who was sitting in the audience, for sharing her story with the world through the movie.
"What this movie did is tell a story about class in America, tell a story about the disenfranchised, tell the story about a woman who was not embraced for her individuality, tell a story about truth and the perception of truth in the media and truths we all tell ourselves. It's an extraordinary movie, and I'm so proud of it," she added.
"It's nice to be in a movie that people see. Thanks to everyone who worked on this film. It's an amazing cast, a very generous (one). I'm forever grateful for this amazing part," he said.
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The actor was all praise for his co-star Francis McDormand, who plays the aggrieved mother of the rape victim in the film.
"... You're a bada**, a force of nature, and it was really fun to be your sparring partner and thanks for making me a better actor. This movie is about compassion and I think we need some of that these days," the actor added.