Pieterse was 13 years old in late 2009, when she was cast in the Freeform teen drama television series "Pretty Little Liars" (2010–2017) as Alison DiLaurentis, the former "queen bee" of her high school clique. She had a regular role in the series from the first four seasons, until later became a main role in the fifth season, with one of the five girls. Since its debut, the series has received mixed reviews from television critics, but remained a relative success for Freeform, garnering a large fandom. Pieterse reprised her role as Alison, in the spin-off series titled "" on Freeform in 2019. On "Pretty Little Liars: The Perfectionists", she portrays the grown-up version of Alison who is working as a teaching assistant at Beacon Heights University.