Grace was born Margaret Grace Denig in Worthington, Ohio, the second of three children to parents Valinn and Rick Denig, who ran a family jewelry business. Her family lived in a 200-year-old house, the first saltbox house in central Ohio. She attended Worthington Christian Schools from kindergarten through ninth grade and briefly attended Thomas Worthington High School, where she began acting in school plays and community theater, including a Jewish Community Center's Gallery Players' 2000 production of The Crucible. Her parents divorced "amicably" when she was 16 years old, but her mother sought a "fresh start". Grace dropped out of high school to move to Los Angeles, California with her mother, while her younger siblings Ian and Marissa continued to live with their father. In Los Angeles, Grace and her mother moved around often as they struggled financially, taking out short-term rents rather than paying for permanent residence while eating a basic diet as it was all they could afford.
Grace acquired an agent within a week of relocating to Los Angeles and enrolled in acting classes. She landed her first role in Rachel's Room, a 2001 web-based video series about the affairs inside a teenage girl's bedroom that was created by Dawson's Creek executive producer Paul Stupin. Her next role was on the 2002 television series Septuplets, which was cancelled before the first episode had aired. Her breakout role was on 2002's television movie Murder in Greenwich, based on the true story of 15-year-old Martha Moxley's murder. She was nominated for a Young Artist Award for her portrayal of Moxley in the Best Performance in a TV Movie, Miniseries or Special – Leading Young Actress category, but lost to Clara Bryant for Tru Confessions. She went on to feature in minor roles on the television series CSI: Miami, The Lyon's Den, Miracles, Like Family, Cold Case and Law & Order: Special Victims Unit, and the films Twelve Mile Road and Creature Unknown, before appearing in a season-long recurring role on Oliver Beene as a Swedish exchange student.
In mid-2004, Grace's agent sent her the script for the pilot episode of Lost; she was given the role of Shannon Rutherford after a successful audition. She was nominated in 2005 for a Teen Choice Award for Choice TV Breakout Performance – Female for her role on Lost, but lost to Desperate Housewives' Eva Longoria. She lived in Hawaii during the filming of the show's first season, and signed on to star opposite Tom Welling in The Fog, a 2005 remake of the 1980 horror film of the same name, as a character originally played by Jamie Lee Curtis. Though the filming of Lost was supposed to have ended before The Fog began, the productions coincided due to Lost's extended season finale and Grace flew between the two sets, on Hawaiian the island of Oahu and on Bowen Island in British Columbia, Canada. After ranking at #27 on Maxim's Hot 100 list of 2005, she returned for Lost's second season. Her character was killed off the series in the season's eighth episode, "Collision" when the series' writers began to feel that the character's "story avenues [were] limited". Executive producer Carlton Cuse said that Grace's departure from the show was "sort of a win-win" as she was eager to enter a full-time career in film. After leaving the series, she nevertheless joined the other principal Lost cast members of season 2 onstage at the 12th Screen Actors Guild Awards where Lost won the award for Outstanding Performance by an Ensemble in a Drama Series.
Variety reported in May 2005 that Grace was in negotiations to play X-Men character Kitty Pryde in the 2006 film X-Men: The Last Stand, but in July, auditions were reportedly being held for her replacement. The role ultimately went to Ellen Page, and Grace later revealed that she had never been contacted about the role and was surprised to read that she was up for the part in question. Grace's next role was in the 2007 independent film Suburban Girl, alongside Sarah Michelle Gellar and Alec Baldwin. In 2007, she starred in The Jane Austen Book Club, based on Karen Joy Fowler's novel of the same name. She is a fan of Jane Austen and had read Fowler's novel when it was released in 2004. When she was given the film's script, she met with the director Robin Swicord, with whom she says she "geeked out", and was given the role of Allegra, an openly lesbian 20-year-old. After the filming of The Jane Austen Book Club was completed, Grace briefly returned to Hawaii to shoot a guest spot on the Lost season 3 episode "Exposé". She starred in the 2008 thriller film Taken with Liam Neeson, who was at the top of a list of male actors Grace wished to work with that she had written just two months before she was cast. She recently played the lead in Simon Fellows' Malice in Wonderland, a modern adaptation of Lewis Carroll's Alice's Adventures in Wonderland. She has begun production on Flying Lessons and appeared opposite Tom Cruise and Cameron Diaz in James Mangold's film Knight and Day. As of September 2010 it has been revealed that she has signed on to play Irina in Breaking Dawn- Part 2.
Grace lived in Honolulu, Hawaii while working on Lost, saying "I love it here, but it's not a place you can really pursue acting." She claimed that, while living in Hawaii, her Lost co-stars would introduce her to men and that some "would probably have me with a different guy every night". She said that her male co-stars were "mostly very protective" and "very opinionated" when deciding whether she should date certain men; actor Josh Holloway once offered to help her select dates from a portfolio of male models with whom he previously worked. Grace and Ian Somerhalder, her onscreen stepbrother, were rumored to be dating in April 2006 after both had left Lost. When asked about Somerhalder in August, she stated, "Ian's great, I adore him, although I'm only 22 – far too young to even think about having a serious relationship." While still working on Lost, Grace and Somerhalder adopted a feral cat named Roo which they found "literally dying" in the jungle on the set. She said that the cat is now her "travel buddy". In 2008 and 2009, she briefly dated Blake Mycoskie, founder of TOMS Shoes.
Grace says that the person she is most inspired by is her mother. When asked about her closest friend, she said that she and her mother were more like sisters and that she is "lucky to have an exceptionally cool mum". She is a self-proclaimed anglophile, having written to a pen pal in the Lake District from the age of eight, having first visited England at 13 years old, and admiring a number of British poets as well as William Shakespeare. She calls herself very clumsy, claiming to "trip over my legs all the time", and was jokingly nicknamed "Maggie Graceless" by one of her former castmates. She plays in a Los Angeles kickball league with her friends.
- Murder in Greenwich as Martha Moxley
- Twelve Mile Road as Dulcie Landis
- Creature Unknown as Amanda
- The Fog as Elizabeth Williams
- Suburban Girl as Chloe
- The Jane Austen Book Club as Allegra Avila
- Taken as Kim Mills
- Malice in Wonderland as Alice
- The Experiment as Bay
- Flying Lessons as Sophie Conway
- Knight and Day as April
- Breaking Dawn - Part 2 as Irina (in-production)
- Lockout as Emilie Warnock
- Taken 2 as Kim Mills
- Decoding Annie Parker as Sarah
- Relative Insanity as Nina
- CSI: Miami as Amy Gorman (2003)
- The Lyon's Den as Haley Dugan (2003)
- Miracles as Hannah Cottrell (2003)
- Like Family as Mary (2004)
- Cold Case as Rene (2004)
- Oliver Beene as Elke (2004)
- Law & Order: Special Victims Unit as Jessie Dawning (2004)
- Lost as Shannon Rutherford (2005-2007, 2010)