[5] Barnes cites his mother's Jewish South African childhood, his father's scientific education, and his attendance at what he felt was a "vaguely Christian" school where he "liked the hymns" as formative influences.

[3][2] Prior to university, he spent two years singing, television presenting, and working with entertainment svengali Simon Fuller to open a jazz club and release an album, both of which did not materialise.

[5] Barnes then studied English literature and drama at Kingston University, appearing and directing in productions such as Don Juan (alongside Katrina Drovandi as the lead), The Golden Age, Exposure and The Zoo Story.

[14][15][5] Barnes made his feature film debut as young Dunstan Thorn in Stardust (2007), directed by Matthew Vaughn and based on Neil Gaiman's novel of the same name.

[17] Adamson described the film as "a coming of age and, to some degree, a loss of innocence story, with Caspian starting out quite naïve, then craving revenge and finally letting go of the vengeance.

[22] In November 2008, Barnes starred in the role of John Whittaker opposite Jessica Biel and Colin Firth in Stephan Elliott's romantic comedy Easy Virtue, based on Noël Coward's play of the same name.

[23] In the psychological thriller Locked In, directed by Suri Krishnamma and released in September 2010, he played Josh, an American father whose daughter seems to be in a coma due to a car accident.

Barnes returned to the West End stage in London with a starring role as World War I soldier Stephen Wraysford in Birdsong, a drama based on Sebastian Faulks's novel of the same title.

[29][30] The Hollywood Reporter says Barnes and his co-star Robert Sheehan "convincingly portray young talents who were in the right place at the right time but made the wrong moves".

[31] In September 2012, Barnes appeared in The Words as a young American soldier and literary genius stationed in France during the end of World War II, who fell in love with a French waitress and whose manuscript gets lost in post-war era.

"[33][34] He then played Alejandro in The Big Wedding, a remake of the original 2006 French film Mon frère se marie (My brother is getting married) released in April 2013.

Variety wrote that with "his male-model features shamelessly lapped up by the camera from the opening frame onwards, [Barnes] is as limber and likeable as he's yet appeared on screen, his native accent convincingly hidden behind a middle-American husk.

Barnes portrayed Billy Russo, a former sniper who started his own private military company Anvil, in The Punisher, a Netflix adaptation set in the Marvel Cinematic Universe, the first season of which aired in November 2017.