Grace was born Margaret Grace Denig in Worthington, Ohio the second of three children of parents Valinn (née Everett) 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 2000 Gallery Players production of The Crucible, at a local Jewish Community Center (though she is not Jewish). Her parents divorced “amicably” when she was 16 years old, and 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 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, dated 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 dated Blake Mycoskie, a contestant on the second season ofThe Amazing Race and 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