Maggie starred in her first film in 2002 with Murder in Greenwich (2002) and then Shop Club (2002). In 2003, at the age of 20, she played an estranged daughter of Wendy Crewson and Tom Selleck in the drama Twelve Mile Road (2003).
Grace's career breakthrough came one year later with Lost, playing Shannon, one of the main characters of a serial drama with an ensemble cast. She was officially credited as part of the main cast from the "Pilot, Part 1" episode through the second season's 7th episode, "The Other 48 Days". In her subsequent appearance in the 8th episode, "Collision", she was billed as "Special Guest Star", and after her character's death, she appeared on Lost in "Exposé". In "The End", she returned to Lost as a main character.