In the summer following his first year of study, He was cast as Lorenzo in the 2004 Al Pacino-vehicle The Merchant of Venice, breaking the school's policy of not allowing students to audition for outside productions.

[10] Subsequently, he decided not to return to school and continued working, appearing in guest spots on TV and supporting roles in movies like the 2005 historical drama Casanova and the 2006 BBC sci-fi film A for Andromeda.

[11] Cox's breakout role was as the main protagonist, Tristan Thorn, in the 2007 fantasy film Stardust, in which he starred opposite Claire Danes.

Also in 2011, he signed on for a recurring role in the second season of the Martin Scorsese-produced HBO original series Boardwalk Empire as Owen Sleater, an Irish enforcer with ties to the IRA.

Cox portrayed Matt Murdock in Marvel's Daredevil TV series as well as the 2017 team-up miniseries event The Defenders, produced and released through Netflix.

In the following years, Cox shared his interest in reprising the role in a future project,[23] also noting his contractual obligations by Marvel Studios to do so.

[24] Between filming seasons of Daredevil, Cox made his New York theater debut, co-starring in the off-Broadway production of Incognito at the Manhattan Theatre Club.

[27] He also acted opposite Michael Caine, Jim Broadbent, Ray Winstone, and others in the 2018 film King of Thieves, based on the true story of the 2015 Hatton Garden jewelry heist in London;[28] it reunited him with James Marsh, who directed him in 2014's The Theory of Everything.

[33] In fall 2020, Cox began filming the RTÉ Dublin crime series Kin, in which he stars with Aidan Gillen, Clare Dunne, Maria Doyle Kennedy, Ciarán Hinds, and Emmett Scanlan.

[36] The first Marvel Studios production he appeared in was Spider-Man: No Way Home (2021),[37] in which Matt Murdock gives legal advice to Peter Parker for ostensibly murdering Mysterio.

[43][44][45][46] At the time of their marriage, they both worked for Marvel Television, where Thomas was Vice President of Original Programming and Cox starred in Daredevil.