Thomason first came to prominence on British television in the fifth (1996) series of "Pie in the Sky" playing the waitress, Sally, in eight episodes. She went on to appear in "Playing the Field" (1998) and "Where the Heart Is" (1998), as well as in the first series of the BBC Three drama Burn It (2003). In the United States, she appeared in Disney's The Haunted Mansion. She played Brandy in the film My Baby's Daddy and Victoria in Black Knight. She also played Vicki in Pure. She starred in the U.S. television series Las Vegas as Nessa Holt in seasons one and two, a total of 47 episodes.
On 21 October 2004, she was a guest on the radio show Loveline. In January 2008, she appeared in Messiah V: The Rapture. In August 2008, Thomason was cast as a series regular in the CW show Easy Money. In late 2009, she appeared on General Hospital. After appearing in the pilot episode for White Collar as agent Diana Berrigan, she returned for the first-season finale. It was announced that she would appear on the series full-time in the second season. She is also the new voice of Diana in Hitman: Absolution.
Thomason had the recurring role of Naomi Dorrit on the television show Lost. Being a friend of Dominic Monaghan (who played Charlie Pace on the series), she was a huge fan of the show and excited to be cast. She first appeared in the third-season episode "Catch-22", which aired on 18 April 2007, and continued to appear until the season finale "Through the Looking Glass". She next appeared in "The Beginning of the End" and in the season-five episode "Some Like It Hoth".
On 5 April 2009, Thomason married lighting technician Craig Sykes in Malibu, California. Their daughter, Tallulah Anaïs, was born on 12 June 2013.
Thomason said about the lack of character transparency in Lost, "That's kind of difficult as an actor, but also freeing in a way, because you just have to play the scenes as they're written. And then if you find out if you're a big villain or whatever, then you play that... you know when that worked really brilliantly? On 24 with Nina Myers. She didn't have a clue – Sarah Clarke, the Actress – and that made it really brilliant, because she played Nina honestly. Really honestly. And then she was a villain. Genius. That was such a great one."