His acting career began when he was approached in a London coffee shop by actor Steven Berkoff, who later appeared alongside Butler in "Attila" (2001/I) (TV), who gave him a role in a stage production of Coriolanus. After that Butler decided to give up law for acting. He was cast as Ewan McGregor's character Renton in the stage adaptation of Trainspotting. His film debut was as Billy Connolly's younger brother in "Mrs. Brown" (1997).

While filming the movie in Scotland, he was enjoying a picnic with his mother, near the River Tay, when they heard the shouts of a young boy who had been swimming with a friend who was in some trouble. Butler jumped in and saved the young boy from drowning. He received a Certificate of Bravery from the Royal Humane Society. He felt he only did what anyone in the situation would have done.

His film career continued with small roles, first in the James Bond movie "Tomorrow Never Dies" (1997) and then Russell Mulcahy's "Tale of the Mummy" (1998). In 2000, Butler was cast in two breakthrough roles, the first being Attila the Hun in the USA Network mini-series "Attila" (2001/I) (TV). The film's producers wanted a known actor to play the part but kept coming back to Butler's screen tests and decided he was their man.

He had to lose the thick Scottish accent, but managed well. Around the time Attila was being filmed, casting was in progress for Wes Craven's new take on the Dracula legacy. Also wanting a known name, Butler wasn't much of a consideration, but his unending tenacity drove him to hounding the producers. Eventually, he sent them a clip of his portrayal of Attila.

Evidently they saw something because "Dracula 2000" (2000) was cast in the form of Butler. Attila's producers, thinking that his big-screen role might help with their own film's ratings, finished shooting a little early so he could get to work on "Dracula 2000" (2000). Following these two roles, Butler developed quite a fan base, an Internet site and began appearing on lists everywhere.

Since then he has appeared in "Reign of Fire" (2002) as Creedy and "Lara Croft Tomb Raider: The Cradle of Life" (2003) as Terry Sheridan, alongside Angelina Jolie. The role that garnered him most attention from both moviegoers and movie makers alike was that of Andre Marek in the big-screen adaptation of Michael Crichton's novel "Timeline" (2003).

Butler played an archaeologist who was sent back in time with a team of students to rescue a colleague. Last year he appeared in Andrew Lloyd Webber's musical "The Phantom of the Opera" (2004), playing the title character in the successful adaptation of the stage musical. It was a role that brought him much international attention. Other projects include "Dear Frankie" (2004), "The Game of Their Lives" (2005) and "Beowulf & Grendel" (2005).

In 2007 he starred as Spartan King Leonidas in the Warner Bros. production "300" (2006), based on the Frank Miller graphic novel, and "Shattered" (2007) co-starring Pierce Brosnan and Maria Bello, which aired on network TV under the title Shattered. He most recently starred in "P.S. I Love You" (2007) with Academy Award-winner Hilary Swank .
In 2007, he appeared in "Nim's Island" (2008) and "RocknRolla" (2008), and recently completed the new Mark Neveldine / Brian Taylor film "Gamer" (2009). His latest film "The Ugly Truth" (2009) co-starring Katherine Heigl began filming in April 2008.