Samaire Rhys Armstrong made her film debut portraying the role of Kara Fratelli, one of the conjoined twins in the Parody film Not Another Teen Movie in 2001. Samaire has since appeared in notable small roles in films such as Would I Lie To You, Gramercy Park and Trash. In 2006 Samaire co-starred alongside Lindsay Lohan in the film Just My Luck portraying the role of Maggie, Lohan's character's best friend. The film was a box office bomb and was negatively received by critics.

In 2005 The O.C. producer McG offered the leading female protagonist role in the horror film Stay Alive. She portrays the role of Abigail, an aspiring photographer and love interest to Jon Foster's character. The film was released on June 29, 2006 and debuted at #3 at the U.S. box office. Despite a worldwide gross of just$27,105,095 it was considered a box office success compared to the film's modest $9 million budget. The film was negatively received by critics with her performance receiving mixed reviews.

In 2006 Armstrong successfully auditioned for the role of Nell Bedworth in the Canadian comedy It's a Boy Girl Thing. The film was released on December 26, 2006 and received positive reviews from critics. Despite its limited release the film managed to be considered a box office success earning worldwide $7,385,434. Samaire's performance in this film was praised by critics.

In 2007 Armstrong starred in a relatively small supporting role as Jenny in the horror film Rise: Blood Hunter alongside Lucy Liu. The film was a box office and critical failure. She also had a leading role portraying Caitlin Atwater in the Lifetime film The Staircase Murders. The film follows the real-life crimes of Michael Peterson who was convicted in 2003 of pushing his wife down the stairs. The film received mixed reviews.

In 2008 Armstrong starred the independent romance film Around June. The film follows a young woman who finds love with an illegal immigrant whose love changes her life. Filming took place in San Francisco, California in 2008. The film played at the Santa Cruz Festival in 2009.. Armstrong has since been cast in another independent film, a crime thriller The Last Harbor. No official release date has been announced as of January 2011. She will also star in another thriller, 5 Souls.

In 2000, Armstrong made her television debut guest starring in an episode of the teen drama television series Party of Five portraying Meredith. Throughout 2000 to 2001, Armstrong made guest roles on high profile television shows such as The X-Files, ER, Judging Amy and Freaks and Geeks.

In 2003, Armstrong was cast in the role of Anna Stern on the Fox teen drama television series, The O.C.. Armstrong was intended to only appear in one episode. But after her character proved to be a fan favorite was brought back to appear in several more episodes. She portrays the role of a self-confessed comic book reader and a sarcastic socialite from Pittsburgh who is a recurring love interest for Seth Cohen. Despite the regular appearance of her character throughout the series first season, she was not promoted to series regular. In 2006, Armstrong was confirmed to return for the series third season for two-episodes. She did not return to the series which was cancelled after its fourth season.

In 2004, Armstrong appeared on the HBO television series, Entourage an assistant to Ari Gold and love interest for Eric Murphy. Critics noted many similarities to her character Anna Stern and Emily. However she was praised for her performance. She appeared in only eight episodes of the series' first two seasons. She went on to guest star in the television series CSI: Miami and Living with Fran.

In 2007, Armstrong was cast in her first series regular role on the ABC drama/comedy television series, Dirty Sexy Money. She portrays the role of Juliet Darling, a spoiled heiress who aspires to be a serious actress. The series was an instant success and premiered on September 26, 2007 to 10.44 million viewers. The series received mixed to positive reviews from critics. Her character has been noticed to have many similarities to real-life heiress Paris Hilton. However in March 2008 Armstrong announced she would not return for the series second season as a series regular. The series did not maintain its success and slipped in ratings throughout its run dramatically. The series was cancelled on August 8, 2009.

In 2010, Armstrong guest appeared in the HBO television series How To Make It In America in the series first episode portraying the role of Jane.

In 2012, she appeared in a The Mentalist episode, "Pink Tops" as a prostitute turned confidential informant for the California Bureau of Investigation. Her character Summer shared a kiss with Tim Kang's character Kimball Cho in the episode "At First Blush" after Cho admitted having feelings for Summer.

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