Darby was born and raised in Kodiak, Alaska. Her father was a commercial crab fisherman. The family later moved to Dutch Harbor in the Aleutian Islands, and, finally, Mercer Island, near Seattle. She attended the University of Puget Sound, graduating in 1993 with a degree in Communications and a minor in theater. She later went back to acting school at the American Conservatory Theater in San Francisco.
She is best-known for her recurring roles as "April Green" on Jericho (2006) and as "Helen Bishop" on the period drama, Mad Men (2007). She stars in the ABC drama, Scandal (2012), as "Abby Whelan". She also makes recurring appearances as Gibbs' deceased wife, "Shannon Gibbs", on NCIS: Enquêtes spéciales (2003).
