In 2004, Stevens began his professional acting career when Peter Hall selected him to play Orlando in touring productions of As You Like It works of Shakespeare. His debut performances won recognition from critics in the United Kingdom and the United States, as well as awards from Ian Charleson Awards. In 2006, Stevens acted as Nick Guest in the BBC adaptation of Alan Hollinghurst's novel, The Line of Beauty. In 2008, Stevens appeared on the BBC adaptation of Jane Austen's novels, Sense & Sensibility, as Edward Ferrars, and West End production of the drama The Vortex by Noël Coward. In January 2009 he appeared in Agatha Christie's Marple: Nemesis at ITV1.
In 2010, Stevens began to succeed after being chosen to play Matthew Crawley in the Downton Abbey series on ITV created by the Oscar-winning scriptwriter, Julian Fellowes. The series became a global sensation and successfully won several Emmy, BAFTA, Golden Globe and Screen Actors Guild Awards nominations since its debut. In November 2011, Stevens hosted an episode of Have I Got News for You. In March 2012 he played in Vamps and Summer in February, a romance film set in the Edwardian era. He made his Broadway debut with Jessica Chastain and David Strathairn in the drama The Heiress.
In 2014, Stevens starred in the film The Guest, where he won praise for his role as a former soldier who carried out a series of murders to protect his true identity. He earned a Saturn Award for Best Actor nomination for his performance in the film. In March 2015, Stevens was announced to play the role of Beast, alongside Emma Watson as Belle, in a Disney adaptation of the classic animated film of the same name, Beauty and the Beast. To carry out this role with motion capture techniques, he had to wear a 40-pound lycra costume with 10-inch rights during the shooting. In February 2016, Stevens was chosen to play the main character David Haller in the Legion series on FX, an X-Men related drama by Noah Hawley. Stevens will then appear in The Man Who Invented Christmas by Bharat Nalluri and The Raid's director's thriller, Gareth Evans, titled Apostle.