Hannigan was born in Washington, D.C. to Albert Hannigan, an Irish American truck driver, and Emilie Posner, a Jewish American real estate agent. 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 in a commercial for the "Duncan Hines cookie mix" in 1978, it wasn't until she moved to Los Angeles, California in 1985 that she formally began her acting career. Living with her mother and attending North Hollywood High School, she successfully auditioned for agents while visiting her father in Santa Barbara.
Career
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. Throughout most of the early 1990s, she appeared in commercials and supporting roles on television shows, including Picket Fences, Roseanne and Touched by an Angel.
In 1997, Hannigan was cast to play Willow Rosenberg, Buffy's best friend, on the television series Buffy the Vampire Slayer. The show became a big success, and Hannigan gained recognition, subsequently appearing in several notable films aimed at teenage audiences, including American Pie, American Pie 2, Boys and Girls, and American Wedding. By the time Buffy had ended in 2003, Hannigan was earning a $250,000 salary for each episode. She also had a guest spot on the Buffy spin-off, Angel, portraying her role of Willow in a few episodes (including most notably "Orpheus", during Buffy's seventh season), but none after Buffy finished production.
In early 2004, Hannigan made her West End debut, starring in a stage adaptation of When Harry Met Sally... at the Theatre Royal Haymarket, opposite Luke Perry. In 2005, she returned to starring in a regular television series, appearing in the currently-running show, How I Met Your Mother, as Lily, and also playing a recurring guest role on Veronica Mars as Trina Echolls.
In February 2006, Date Movie, in which Hannigan stars as a character meant to be a parody on romantic comedy heroines, was released in theaters.
Hannigan is currently playing Lily, a New York kindergarten teacher and aspiring artist in the CBS situation comedy "How I Met Your Mother".
Hannigan recently presented a Grammy at the 2007 Grammy Awards.
