She is best known for her roles as Corrina Mallon in the RTÉ drama Clean Break (2015) and Emma in the film Life's a Breeze (2013), which earned her an IFTA nomination for Best Actress.

The series, directed by Gillies MacKinnon and Damien O’Donnell, told the story of Frank Mallon, Corrina's father and a car dealer.

The film was written and directed by Megan K. Fox and told the story of a "teenager finding strength and happiness in music in the face of her battle with depression".

Thornton gained further recognition for her part in the 2016 film Sing Street, playing the role of Ann Lawlor, the older sister of the main protagonist Conor.

[7] Thornton has since starred in three short films: Inside I'm Racing in 2017, Jack Reynor's Bainne opposite Will Poulter and Ciarán Dooley's Twin, both in 2019.