Breakthrough
In 2017, she rose to prominence after co-starring as one of the lead characters, Beverly Marsh, in the horror film It, an adaptation of Stephen King's novel of the same name. Directed by Andy Muschietti, It also starred Jaeden Martell, Finn Wolfhard, Wyatt Oleff, Bill Skarsgård, Jack Dylan Grazer, Jeremy Ray Taylor, and Chosen Jacobs. Later that year, she played one of Kristen Bell and Dax Shepard's children in Sia's music video "Santa's Coming for Us".In 2018, Lillis appeared in the HBO television miniseries Sharp Objects, in which she played a young Camille Preaker. In 2019, Lillis starred as teenage sleuth Nancy Drew in the Warner Bros. film adaptation of Nancy Drew and the Hidden Staircase. While the film received mixed to positive reviews, Lillis's performance was singled out for praise. She reprised her role as Beverly Marsh in It: Chapter Two in 2019.
In 2020, Lillis portrayed Gretel in Gretel & Hansel, a re-imagining of the German fairy tale "Hansel and Gretel". The following month, Lillis played the lead role in the Netflix original series I Am Not Okay with This. In November, she appeared with Paul Bettany in Uncle Frank, an independent film by True Blood and Six Feet Under creator Alan Ball, which premiered on Amazon Prime.
In October 2022, Lillis performed in Heroes of the Fourth Turning at Studio Theatre in Washington, D.C..
Lillis appeared in the 2023 film Dungeons & Dragons: Honor Among Thieves. She was cast as the younger sister of Michael Cera's character, Eric, in the Dustin Guy Defa film The Adults. Lillis is part of an ensemble cast, including Jeff Goldblum, Tom Hanks, Maya Hawke, and Scarlett Johansson, who appear in the 2023 Wes Anderson film Asteroid City.
In July 2024, she was cast in the Peacock television series All Her Fault. She starred alongside Dave Bautista in the 2025 action film Trap House.
