The actress, 28, walked the red carpet with her father and fellow nominee Peter Gallagher, who helped her celebrate her best featured actress in a musical nod for her performance as Bella Fox in Jagged Little Pill.
She put on a gorgeous display in a red custom Derek Lam 10 Crosby dress with a thigh-high leg slit, matching her bold red lip, as she accessorized with a pair of strappy silver stilettos and a jeweled clutch. Peter, 66, walked his daughter down the carpet in a classic black tuxedo.
Kathryn was nominated in the same category as her costars Celia Rose Gooding and Lauren Patten. Patten won the award, also beating out Robyn Hurder for Moulin Rouge! The Musical and Myra Lucretia Taylor for Tina — The Tina Turner Musical. Kathryn later performed onstage, alongside Gooding, Patten and the rest of the Jagged Little Pill cast.
The You actress previously celebrated her nomination on Instagram in October. "For the last three years I've had the pleasure of working alongside the finest artists I could have ever dreamed of making a show with and that in and of itself has felt like winning the lottery," she wrote.
"This is a very cool feeling I didn't think I would ever feel and I'm just really grateful you all wanted to see our play and I can't wait until Broadway is back at it," Kathryn continued. "Going to call my dad back now."
Her father was previously nominated for a Tony Award in 1986, when his performance as Edmund Tyrone in Long Day's Journey Into Night earned him a nod for best featured actor in a play.
Jagged Little Pill, which led the nominees with 15 nods at Sunday's awards show, is a jukebox musical, featuring music from Alanis Morissette's 1995 album of the same name. The show follows the Healy family as the navigate a number of social issues in their suburban Connecticut community.
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The show has recently faced scrutiny over the alleged "harm" inflicted by the producers on the transgender and nonbinary community. Gooding and actor Antonio Cipriano called out the producers, while announcing that their performance with the cast at the Tony Awards would be their last.
The lead producers issued a social media statement on Saturday ensuring a "comprehensive investigation" by a third party.