At age four, Hannigan began appearing in commercials. She moved to Hollywood at age 11. Living with her mother and attending North Hollywood High School, she successfully auditioned for agents while visiting her father in Santa Barbara. After attending North Hollywood High School, she attended California State University, Northridge where she earned a degree in Psychology.
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.
In 1997, Hannigan was cast to play Willow Rosenberg on the television series Buffy the Vampire Slayer. The show became a 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 ended in 2003, Hannigan was earning a US$250,000 salary for each episode. She also had a guest spot on the Buffy spin-off Angel, reprising her role of Willow in a few episodes, 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 Theatre Royal Haymarket, opposite Luke Perry. In 2005, Hannigan returned to starring in a regular television series, appearing in the hit comedy, How I Met Your Mother, as Lily, and also playing a recurring guest role on Veronica Mars as Trina Echolls. In February 2006, Hannigan starred as Julia Jones in Date Movie, a parody of romantic comedies. She was also a guest star on the ABC animated sitcom The Goode Family in 2009.
In 2012, Hannigan reprised the role of Michelle in American Reunion. Since 2016, Hannigan has hosted the television series Penn & Teller: Fool Us. Hannigan played Ann Possible, Kim's mother, in the Disney Channel original movie Kim Possible (2020), based on the animated series.
Alyson Hannigan is one of the only two characters to appear in every 144 episodes of Buffy the Vampire Slayer along with Buffy Summers herself Sarah Michelle Gellar.
- Buffy the Vampire Slayer season 1
- Buffy the Vampire Slayer season 2
- Buffy the Vampire Slayer season 3
- Buffy the Vampire Slayer season 4
- Buffy the Vampire Slayer season 5
- "There's No Place Like Plrtz Glrb"
- Buffy the Vampire Slayer season 6
- Buffy the Vampire Slayer season 7
Hannigan also appears in the following featurettes:
- "Season 6 Overview"
- "Buffy Goes to Work"
- "Behind the Scenes of 'Once More, with Feeling'"
- "Season 7 Overview"
- "Buffy Wraps"
Behind the scenes for Buffy the Vampire Slayer and Angel.
- Alyson Hannigan on Wikipedia
- Stephen M. Silverman, "Buffy's Pals Alyson, Alexis Get Married". PEOPLE.com, October 14, 2003.