She played small roles on Seinfeld (as a doomed customer of The Soup Nazi) as well as on the shows Party of Five, Frasier, Hope & Gloria, and NYPD Blue.

Among her most popular characters on the show were high school music teacher Bobbie Mohan-Culp; Margaret Jo McCullen, the National Public Radio host of Delicious Dish; folk singer Cinder Calhoun who sang feminist songs at Lilith Fair, and her impressions of Martha Stewart and Celine Dion.

In 2000, she and Chris Parnell auditioned to succeed Colin Quinn as co-anchors of the news parody segment Weekend Update, but the positions went to Tina Fey and Jimmy Fallon.

In 2004, Gasteyer played the mother of the main character Cady Heron (Lindsay Lohan) in the feature film Mean Girls, written by SNL castmate Tina Fey.

[12][13] On June 24, 2005, Gasteyer originated the role of Elphaba in the Chicago sit-down production of the musical Wicked, alongside Kate Reinders as Glinda.

[18] She starred as Kitty Dean in the Broadway play The Royal Family which began performances at the Samuel J. Friedman Theatre on September 15, 2009, and officially opened on October 8, 2009.

[23] From 2011 to 2014, Gasteyer co-starred as Sheila Shay on the ABC comedy series Suburgatory, and, as of March 2013, she was a spokeswoman for the Weight Watchers diet plan.

In February 2020, it was announced that Gasteyer will play Katherine Hastings in the NBC comedy pilot American Auto, written by Justin Spitzer.

[32] In September and October 2020, she voiced a red panda named Nut in an adult animated series titled Magical Girl Friendship Squad.

[33][34] Gasteyer stars as Queen Aggravain alongside Sutton Foster, Will Chase, Brooks Ashmanskas, Daniel Breaker, and Michael Urie in a Broadway revival of Once Upon a Mattress.