She was one of the original cast members of Mr. Show, and also had roles on The Larry Sanders Show and Veronica's Closet.

Recently she has ventured into more dramatic roles, most notably her role as CTU tech analyst Chloe O'Brian on 24, which she joined in 2003 at the start of the show's third season.

Her character was a hit with viewers and critics and was one of the few cast members to return in the show's fourth season. After being a regular guest star for one season, Rajskub became a main cast member in the show's fourth season and has continued into the fifth season, which started in January 2006.

She also appeared in Kelsey Grammer's The Sketch Show on Fox Television and numerous films including Serial Slayer (aka Claustrophobia), Mysterious Skin, Legally Blonde 2, Sweet Home Alabama, Dude, Where's My Car?, Man on the Moon, Punch-Drunk Love, The Anniversary Party, and Firewall, as well portraying a blind girl in Road Trip.

She is also a skilled guitar player, and was part of a comic duo called Girls Guitar Club. She also played guitar and sang for a few minutes of a Gilmore Girls episode which originally aired on May 9, 2006.

Rajskub dated David Cross while on Mr. Show during the first two seasons, but left the show after the relationship ended. She dated Jon Brion for five years until the spring of 2002, and she was briefly engaged in 2005 to Duncan Trussell.