Doubleday first appeared in a commercial for Goldfish crackers at age eight and had a small role in the film Legend of the Mummy (1998). Her parents insisted that she finish high school before pursuing a career in acting. Doubleday was cast in the pilot episode of United States of Tara, a television series created by Diablo Cody. Doubleday played 15-year-old Kate, the daughter of Toni Collette's character. Doubleday was replaced by Brie Larson when the series' creative team chose to go in a different direction with the character. She appeared in the short film 18 in 2009, about a girl dealing with the end of her mother's life.
Doubleday starred opposite Michael Cera in the teen comedy Youth in Revolt (2009), based on the 1993 novel of the same name by C.D. Payne. Doubleday described her character as "mean" and "really complex" to the Los Angeles Times. She plays Sheeni Saunders, an imaginative girl with a dreary life who meets Nick Twisp (Michael Cera) while on a family vacation. The film, directed by Miguel Arteta, premiered at the 2009 Toronto International Film Festival.
Doubleday appeared regularly in the ABC network comedy Mr. Sunshine, a midseason replacement for the 2010–2011 season.
She played the popular and monstrous Chris Hargensen in the 2013 adaptation of Carrie. Doubleday dyed her hair brown for the role of Chris, who is a natural brunette in the novel.
In May 2015, Doubleday appeared in the USA Network pilot for the TV series, Mr. Robot, starring Rami Malek and co-starring Christian Slater. Doubleday plays Malek's character's childhood friend and current co-worker. The first episode of the series premiered on June 24, 2015.
