In June 2010, The CW announced Cassidy would have a major recurring arc in the fourth season of the teen drama television series Gossip Girl. Cassidy features as Juliet Sharp who attends Columbia and sets out to ruin the life of the show's central character Serena van der Woodsen. Cassidy made her first appearance in the season premiere and went on to appear in over 11 episodes of the season.
In March 2011, Cassidy was cast in the ABC pilot Georgetown. Cassidy was cast as Nikki, a smart and quick-witted junior staffer in the White House Communications Office with connections to the First Lady. The pilot was created by Gossip Girl creator Josh Schwartz. The series was not picked up by ABC.
In 2012, Cassidy was cast in The CW series Arrow, which is based on the Green Arrow comic books, as the female lead Laurel Lance, also known as the superhero Black Canary. The series also stars Stephen Amell and Willa Holland and premiered in North America on The CW on October 10, 2012.
In 2006, Cassidy made her film debut as Tiffany Madison in the horror film When a Stranger Calls. Cassidy has gone on to have notable supporting roles in films Click, Taken and Live!.
In February 2006, Cassidy was cast as Kelly Prelsey in the Dimension Films horror-slasher film Black Christmas, a remake of the 1974 film. The movie centers on an escaped maniac who returns to his childhood home on Christmas Eve, which is now a sorority house, and one by one kills all sorority sisters. The film was released on December 15, 2006 to negative reviews from critics. The film was a moderate success making over $50 million worldwide.
In April 2010, Cassidy appeared in another remake; this time the iconic horror classic A Nightmare on Elm Street, originally released in 1984 of the same name. Cassidy portrayed the role of Kris Fowles; a high school student who becomes a victim to Freddy Krueger. Filming begin in Chicago with the film earning over $31 million in its opening weekend.Cassidy was nominated for a Teen Choice Award in the "Horror/Thriller Actress" category for her role, which she lost to Megan Fox.
Shortly after the release of A Nightmare on Elm Street, Variety announced Cassidy was cast in the Adventure Comedy film Monte Carlo. The film, which also stars Leighton Meester and Selena Gomez, follows three friends who pose as wealthy socialites whilst on holiday in Monte Carlo. Production began in May 2010 with filming taking place in Monaco, Budapest, Paris and Texas. The film was released on July 1, 2011 and received mixed to negative reviews from critics and went on to gross $39 million worldwide.
In May 2012, Cassidy was cast as the title character in the film adaptation of The Scribbler alongside Eliza Dushku, Michelle Trachtenberg and Gina Gershon. She plays Suki, a young woman with multiple personality disorder.