Justin Kirk was born on May 28, 1969, in Salem, Oregon, United States. He grew up in Union, Washington, where he attended a grade school on an Indian reservation. He moved to Minneapolis when he was 12 years old, and then to New York City after graduating from high school. In New York City, he completed a two-year conservatory acting program at Circle in the Square.
His first role on Broadway was a play called Any Given Day, which was performed at the Longacre Theatre. He also appeared in Love! Valour! Compassion! both on-screen and on stage, for which he received an Obie Award for Distinguished Performance in the Ensemble. In addition he won a Back Stage West Garland Award for Outstanding Performance on his role of a piano prodigy in Old Wicked Songs, staged at New York's Promenade Theatre and Los Angeles' Geffen Playhouse.
He currently lives part-time in New York City, and part-time in Los Angeles.