Born and raised in Sacramento, California, Chastain developed an interest in acting from a young age. In 1998, she made her professional stage debut as Shakespeare's Juliet. After studying acting at the Juilliard School, she was signed to a talent holding deal with the television producer John Wells. She was a recurring guest star in several television series, including Law & Order: Trial by Jury. She also took on roles in the stage productions of Anton Chekhov's play The Cherry Orchard in 2004 and Oscar Wilde's tragedy Salome in 2006.