Mia Kirshner (born January 25, 1975) is a Canadian actress, writer, and social activist. She is known for television roles as Mandy in 24 (2001–2005), as Jenny Schecter in The L Word (2004–2009), as Amanda Grayson in Star Trek: Discovery (2017–2019) and Star Trek: Strange New Worlds (2023), and as Isobel Flemming in The Vampire Diaries (2010–2011). Her film credits include Love and Human Remains (1993), Exotica (1994), The Crow: City of Angels (1996), Mad City (1997), Not Another Teen Movie (2001), and The Black Dahlia (2006).

Kirshner was born in Toronto, Ontario, Canada, the daughter of Etti (Henrietta), a teacher, and Sheldon Kirshner, a journalist who wrote for The Canadian Jewish News. Kirshner is a granddaughter of Holocaust survivors; her father was born in the displaced persons camp at Bad Reichenhall in Germany in 1946; and met Kirshner's mother, a Bulgarian Jewish refugee (of Sephardi Jewish descent), after they immigrated to Israel. Kirshner's paternal grandparents were Polish Jews (of Ashkenazi Jewish descent). Kirshner had a middle class upbringing, and attended Forest Hill Collegiate Institute but later graduated from Jarvis Collegiate Institute. Kirshner studied Russian literature and the 20th-century movie industry at McGill University in Montreal. Her younger sister, Lauren Kirshner, a writer, was involved in the I Live Here project.

Kirshner started her career in 1989 in "Loving the Alien", a second-season episode of War of the Worlds, as both Jo, a young resistance fighter who is captured and duplicated by the enemy aliens, and her doppelgänger. Kirshner made her film debut in 1993 at the age of 18 in Denys Arcand's Love and Human Remains. She convinced her father to sign a "nudity waiver" to play a dominatrix. The following year, she starred as a conflicted exotic dancer in Atom Egoyan's Exotica. In 1996, she appeared in The Crow: City of Angels. She also played Kitty Scherbatsky in the 1997 version of Anna Karenina.

Kirshner also appeared in the first three episodes of 24 as the assassin Mandy in 2001. She would later reprise the role for the second season's finale and in the latter half of the show's fourth season. Also in 2001, Kirshner played Catherine Wyler, "The Cruelest Girl in School", in Not Another Teen Movie. The character is primarily a spoof of Kathryn Merteuil (played by Sarah Michelle Gellar) in Cruel Intentions, and was partially based on Mackenzie Siler (played by Anna Paquin) from She's All That. In Marilyn Manson's music video for "Tainted Love", which was featured on the movie's soundtrack, she made a cameo appearance as her character Catherine Wyler.

In 2004, Kirshner was cast as writer Jenny Schecter, a main character in the drama series The L Word. She remained with the show through 2009, for all six seasons.

In 2006, she starred in Brian De Palma's The Black Dahlia in which she plays the young aspiring actress Elizabeth Short, whose mutilation and murder in 1947 remains unsolved. While the film itself was critically panned, many reviews singled out her performance for acclaim. Stephanie Zacharek of Salon.com, in a largely negative review, notes that the eponymous character was "played wonderfully by Mia Kirshner..." Mick LaSalle wrote that Kirshner "makes a real impression of the Dahlia as a sad, lonely dreamer, a pathetic figure." J. R. Jones described her performance as "haunting" and that the film's fictional screen tests "deliver the emotional darkness so lacking in the rest of the movie." In 2010, Kirshner co-starred in the film 30 Days of Night: Dark Days which began filming in the fall of 2009. In 2010, she was cast as Isobel Flemming, a guest role on The Vampire Diaries.

In 2011, she voiced the title character in Bear 71, a National Film Board of Canada web documentary that premiered at the Sundance Film Festival.

On April 20, 2012, it was announced that Kirshner would join the new Syfy series Defiance.