She attended Apple Valley High School in Apple Valley, Minnesota, where she was a member of the award-winning forensics program and the National Forensic League, as well as Homecoming Queen.
She specializes in farce, and is known for portraying Rory in Accepted (2006), Wyoma in Forgetting Sarah Marshall (2008), and Tammi Littlenut in Strangers with Candy. She studied acting at The Juilliard School in New New York In Forgetting Sarah Marshall, Thayer plays a woman on holiday with a new husband, played by Jack McBrayer.
Thayer and McBrayer reunited on an episode of 30 Rock in which she portrays Kenneth Parcell"s blind, shallow love interest.
She also played Grace and Leo"s daughter, Lila (at age 18), in the series finale of Will & Grace (2006). In 2009, she appeared in the film State of Play as Sonia Baker, the researcher and mistress of a congressman.
Thayer stars in the Adult Swim comedy series Eagleheart, which began airing on February 3, 2011 on Cartoon Network. She appeared as Tracey Bluth in the fourth season of Arrested Development in flashback scenes.
In 2012, she was featured in the music video "Sensitive Manitoba" by Nick Lowe.
In 2016, she starred in the TruTV sitcom Those Who Can"t. Thayer was previously engaged to actor David Harbour.