Hannigan was born in Washington, D.C. and is the only child of Emilie Haas and Al Hannigan. Her father is of Irish descent and her mother is Jewish. Her parents divorced when she was two and she was raised mostly by her mother in Atlanta, Georgia.
Hannigan began acting at the age of four in commercials. She later moved to Hollywood and began auditioning for agents. She landed her role on Buffy the Vampire Slayer in 1997 and her career took off from there.
