[8] Following a guest appearance on the Disney Channel series Hannah Montana in 2008,[7] she co-starred with China Anne McClain in the film Jack and Janet Save the Planet, an unaired pilot.
[14] Also in 2011, McCormick had her breakthrough when she was cast in the role of Olive Doyle, a girl with an eidetic memory, in the Disney Channel series A.N.T.
[16] From 2011 to 2014, McCormick had a recurring role on the Disney Channel sitcom Jessie as Connie, a girl obsessed with one of the main characters, Luke.
[18] She had been hired by director Adam Egypt Mortimer to appeal to her then younger fanbase, stating that the film would be "the first intense horror movie experience that some of these kids will ever have".
[23] In 2019, McCormick co-starred with Jake Horowitz in the science fiction film The Vast of Night,[24] based on the Kecksburg UFO incident and the Foss Lake Disappearances.
She starred as Fay Crocker, a young switchboard operator in high school in the fictional town of Cayuga, New Mexico.
[25] McCormick said that she was convinced to play the role after reading the script, where she then met with the film's director Andrew Patterson and auditioned.
[36] In October 2020, it was announced McCormick would star alongside Vinessa Shaw in the upcoming thriller film We Need to Do Something;[37] she was cast as Melissa, who is part of a family trapped in their bathroom during a tornado.
[38] She stated that "I felt really antsy being cooped up in my apartment, and I'm sure everyone can relate to that feeling ... when I read the script, I was struck by the nihilistic tone the movie takes, and I'm a big fan of that.
[43] IGN wrote that "Sierra McCormick brings wide eyes and a sulking snarl to teen daughter Melissa, whose internal drama is signaled by a goth wardrobe topped by a bubblegum pink wig.
[45][46] Her role as Scarlett was praised;[47][48] CBR described her as having a "captivating screen presence", and went on to state that she made "a strong debut ... it'd be seriously surprising if she doesn't eventually get cast in some of Murphy's other projects.
"[49] In 2023, McCormick starred as Sybil in the acclaimed crime thriller film The Last Stop in Yuma County, directed by Francis Gallupi.