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.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.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. Armstrong was intended to only appear in one episode.The film played at the Santa Cruz Festival in 2009.
In September 2013, she played the mother of the blond boy who perpetrated the school shooting in the Sons of Anarchy season six premiere "Straw".In 2007 Armstrong starred in a relatively small supporting role as Jenny in the horror film Rise: Blood Hunter alongside Lucy Liu. 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. 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.Critics noted many similarities to her character Anna Stern and Emily. She appeared in only eight episodes of the series' first two seasons.