She is known for her roles as McKenna Brooks in An American Girl: McKenna Shoots for the Stars,[2][3] as Summer on the Nickelodeon television series School of Rock,[4][5] as Lexie Richardson on the Hulu drama television miniseries Little Fires Everywhere, and as Grace Sullivan on the ABC series Big Sky.

[1] Prior to her film and television career, she performed alongside a local children's song and dance troupe from the age of seven.

[6][7] Pettyjohn has also played various roles in several television series, such as in Revolution, Criminal Minds: Suspect Behavior,[8] Grimm, The United States Of Tara, The Mentalist, and Pure Genius.

[15][16] In 2019 she was cast in the Hulu television series Little Fires Everywhere playing Lexie Richardson, the daughter of Reese Witherspoon's character.

[17] In June 2020, it was announced that Pettyjohn had been cast in the ABC television drama series Big Sky, playing Grace Sullivan who is kidnapped along with her older sister Danielle (Natalie Alyn Lind).