She starred as Kris Fowles in the slasher film remake A Nightmare on Elm Street (2010) and had a recurring role as Juliet Sharp during the fourth season of the teen drama Gossip Girl (2010–2012).

During her time there, she signed a recording contract, releasing a cover of "I Think I Love You", a song which had been a hit for her biological father as part of the soundtrack to his TV show The Partridge Family.

In October 2008, Variety announced Cassidy had been cast on the CBS horror mystery limited series Harper's Island.

The series centered on a group of family and friends who gather for a wedding on an island known for its sinister past full of gruesome murders.

Despite mixed reviews, Cassidy's performance was praised by critics and was cited as "the best thing about the Melrose reboot" by New York magazine.

[15] In April 2010, Cassidy appeared in another remake; this time the horror classic A Nightmare on Elm Street,[16] originally released in 1984 of the same name.

Cassidy portrays the false protagonist Kris Fowles; a high school student who becomes a victim to Freddy Krueger.

[19][20] Shortly after the release of A Nightmare on Elm Street, Variety announced Cassidy had been 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 while on holiday in Monte Carlo.

[21] In June 2010, The CW announced Cassidy would have a major recurring arc in the fourth season of the teen drama series Gossip Girl.

Cassidy portrayed Juliet Sharp, who attends Columbia University and sets out to ruin the life of the show's central character Serena van der Woodsen.

[22] In May 2012, Cassidy was cast as the title character in the film adaptation of the graphic novel The Scribbler, alongside Eliza Dushku, Michelle Trachtenberg and Gina Gershon.

[23] In March 2011, Cassidy was cast in the ABC drama pilot Georgetown as Nikki, a smart and quick-witted junior staffer in the White House Communications Office with connections to the First Lady.

[27] Cassidy left the show as a series regular after her character was killed off in episode 18 of the fourth season on April 6, 2016.

[28] On May 17, 2016, she guest-starred in the Arrow verse spin-off series The Flash for the second time, but as the metahuman Black Siren, Laurel's Earth-2 doppelgänger,[29] in the second-season episode "Invincible".

On July 23, 2016, it was announced that Cassidy had signed a deal with Warner Bros. to appear across multiple CW Arrowverse shows.

[30] On March 27, 2017, it was announced that Cassidy would return to Arrow as a series regular for season 6; reprising the role of Black Siren.