British actor Benjamin Thomas Barnes was born in London, England, to Patricia (Becker), a relationship therapist, and Thomas Barnes, a professor of psychiatry. He has a brother, Jack. His mother is from South Africa, and is of Jewish heritage, while his father is English.
Barnes studied at Homefield Preparatory and King's College, both independent all boys' schools. He began acting in musical theatre, including at the National Youth Music Theatre, and was a member of Hyrise, a boy band. In 2004, he graduated with BA Honours from Kingston University, where he studied drama and English literature.
In 2006, Barnes played Dakin in The History Boys on stage, and made his television debut on the series Doctors (2000). His first film was Matthew Vaughn's Stardust, le mystère de l'étoile (2007), and his second major film role was the epic Le monde de Narnia: chapitre 2 - Le prince Caspian (2008), in which he played Prince Caspian. He next co-starred with Jessica Biel in Un mariage de rêve (2008), played the title role in Ol Parker's Dorian Gray (2009), and reprised his role, now as King Caspian, in Le monde de Narnia: L'odyssée du passeur d'aurore (2010).
In 2011, he co-starred with Robert Sheehan, playing brothers, in Killing Bono (2011). He also had major roles in the films The Words (2012) and Un grand mariage (2013).
In 2015, Barnes played the title role in the fantasy adventure Le septième fils (2014), opposite Jeff Bridges and Julianne Moore. The same year, he portrayed American founding father Sam Adams in the History Channel mini-series Sons of Liberty (2015).
