Cassidy had her breakthrough in 2012, after being cast as Laurel Lance / Black Canary, one of the main protagonists on the television series Arrow, which spawned an entire franchise known as the Arrowverse. Cassidy reprises her role in The Flash, Legends of Tomorrow and voiced the character in the animated web series Vixen. Following her character's death, Cassidy began playing Laurel Lance / Black Siren, an antagonistic version of her character from a parallel universe, starting in The Flash and continuing on to Arrow. Because of her roles in numerous horror films (most of which are remakes of classics of the genre) which include her feature film debut When a Stranger Calls (2006), The Lost (2006), Black Christmas (2006), and A Nightmare on Elm Street (2010), she has been called a modern-day scream queen.
Cassidy was born in Los Angeles, the only daughter of actor and singer David Cassidy and Sherry Williams in 1986. Cassidy's paternal grandparents were actors Jack Cassidy and Evelyn Ward, and she is the niece of teen idol and television producer Shaun Cassidy and actor Patrick Cassidy. Cassidy has a younger half-brother named Beau, born in 1991, from her father's third marriage to Sue Shifrin. In her youth, Cassidy was on the competitive cheerleader team, the California Flyers. A resident of Bell Canyon, California she attended Calabasas High School.
In 2003, Cassidy made her television and acting debut appearing in an episode of the Lifetime drama series The Division. Cassidy has continued to guest appear in television shows such as Celebrity Apprentice 4, Listen Up! and New Girl. In 2005, Cassidy appeared in a recurring role on The WB's series 7th Heaven over the course of four episodes. That same year, she also appeared on the UPN teen drama series Sex, Love & Secrets.
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 Kelli Presley in the Dimension Films horror-slasher film Black Christmas, a remake of the 1974 film. The film 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 2007, Cassidy appeared on The CW's supernatural drama series Supernatural. Cassidy played the role of Ruby, a demon. Making her debut in the show's third season, Cassidy appeared in over six episodes. In 2008, though her character returned for the show's fourth season, Cassidy was replaced by actress Genevieve Cortese.
In October 2008, Variety announced Cassidy had been cast on the CBS horror mystery limited series Harper's Island. Cassidy portrayed the role of Patricia "Trish" Wellington, a bride-to-be. 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. In each episode, at least one character is killed off. Cassidy and her fellow cast members were not told about their characters' deaths until the day they received a script. The series premiered on April 9, 2009 to 10.21 million viewers. Despite ratings failing to maintain the pilot's success, CBS aired the remainder of the series with the series finale airing on July 11, 2009.
In February 2009, The CW cast Cassidy as a lead in the drama series Melrose Place, a reboot of the 1990s series of the same name. Cassidy portrayed the role of Ella Sims, a publicist who resides in a West Hollywood apartment complex and centers on its residents. The series premiered on September 8, 2009 to 2.31 million viewers. 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. The CW canceled the series in May 2010 due to low ratings.
In April 2010, Cassidy appeared in another remake; this time the horror classic A Nightmare on Elm Street, 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. 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 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. 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 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. Cassidy made her first appearance in the season premiere and went on to appear in over 11 episodes of the season.
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. She plays Suki, a young woman with dissociative identity disorder.
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. The pilot was created by Gossip Girl executive producers Josh Schwartz and Stephanie Savage. The series was not picked up by ABC.
In February 2012, Cassidy landed the lead role of Laurel Lance/Black Canary in The CW's series Arrow, which is based on the Green Arrow comic books. The series premiered on October 10, 2012. 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. On May 17, 2016, she guest-starred in the Arrowverse spin-off series The Flash for the second time, but as the metahuman Black Siren, Laurel's Earth-2 doppelgänger, 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 shows, including Arrow, The Flash, and Legends of Tomorrow.
In 2016 Cassidy portrayed late fashion model and actress Sharon Tate in the horror film Wolves at the Door, loosely based on the Manson Family's murders.
On March 27, 2017, it was announced that Cassidy would be returning to Arrow as a series regular for the show's sixth season, but that she would be portraying Black Siren.
In 2002, Cassidy recorded a cover version of "I Think I Love You", which had been a hit for her father's made-for-television band, the Partridge Family. She was 15 at the time, five years younger than her dad when he recorded his version. It was her only song she released. In his 2007 autobiography, David Cassidy said he did not support his then-estranged daughter's attempt at a music career, saying she was too young and had not yet "paid her dues".
Cassidy is a spokeswoman for the charity H.E.L.P. Malawi. In 2016, Cassidy began dating Matthew Rodgers. The couple announced their engagement on June 5, 2017.