British actor Benjamin Thomas Barnes was born in Cambridge, 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 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 (2007), and his second major film role was the epic The Chronicles of Narnia: Prince Caspian (2008), in which he played Prince Caspian. He next co-starred with Jessica Biel in Easy Virtue (2008), played the title role in Ol Parker's Dorian Gray (2009), and reprised his role, now as King Caspian, in The Chronicles of Narnia: The Voyage of the Dawn Treader (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 The Big Wedding (2013).

In 2015, Barnes played the title role in the fantasy adventure Seventh Son (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).