Lancaster has also co-starred in the NBC comedy-spy series Chuck, as the title character's sister, Ellie Bartowski, since the show's inception.

Lancaster was born and raised in Overland Park, Kansas, with a younger brother, Daniel. Her parents, Barbara and Michael, were a homemaker and a real estate agent, respectively, before relocating to Mission Viejo, California, where Sarah took up film/TV acting classes with R. J. Adams at The Actors Workshop.

Her skill impressed a talent agent, who helped the then-eighth grader to secure her first series-regular role on Saved by the Bell: The New Class in 1993. During this time, she was tutored on the set and took college courses at the University of California, Irvine to expedite her graduation from high school, after which she moved to Los Angeles to further her acting career.

She has appeared on Sabrina the Teenage Witch, Dawson's Creek, That '70s Show, Scrubs, and CSI: Crime Scene Investigation. She also landed two recurring roles: on The WB series, Everwood, as Ephram's love interest Madison Kellner; and on David E. Kelley's FOX series, Boston Public.

Lancaster has also worked as a jealous girlfriend-turned-serial killer in the movie Lovers Lane, and as the blackjack dealer, Veronica Harold, on CBS's short-lived Dr. Vegas.