Hannigan was born in Washington, D.C., the daughter of Emilie Posner, a real estate agent, and Al Hannigan, a truck driver. Hannigan is Jewish on her mother's side, and also has Irish ancestry. Her parents divorced a year after her birth and she was raised mostly by her mother in Atlanta.

Although Hannigan had appeared in an industrial film for "Active Parenting" as a baby, as well as starred Read more...

Hannigan's first major film role was in My Stepmother Is an Alien, a science fiction comedy released in 1988; one of her co-stars in the film was actor Seth Green, who would later join her in the regular cast of Buffy as her on-screen boyfriend. Then in 1989, her first regular role on a TV series came when she was cast in the short-lived ABC sitcom Free Spirit. Throughout most of the early-1990s, Read more...

She married actor Alexis Denisof (who played Wesley Wyndam-Pryce in the Buffy/Angel franchise) at Two Bunch Palms Resort in Desert Hot Springs, California on October 11, 2003. They bought a house together in Santa Monica, California.