Griffin began performing in the early 1980s Los Angeles improv comedy troupe, The Groundlings. In an E! True Hollywood Story segment, she stated that she often went to see shows at the Groundlings before she joined. She said one time she was there she went backstage and talked with a Groundling member (Phil Hartman) and asked him what The Groundlings were all about. This led to her taking classes there and eventually being asked into the Main Company. She went on to perform stand-up comedy and teamed with fellow Groundlings alum Janeane Garofalo on the comedy act "Hot Cup of Talk,"[2] later the title of Griffin's 1998 solo HBO special. She did some acting, breaking into film with the supporting role of Connie in the horror movie The Unborn (1991), starring Brooke Adams.

Griffin gradually amassed such TV and film credits as a role in comedian Julie Brown's Medusa: Dare to Be Truthful (1992), a Showtime parody of the backstage film Truth or Dare (1991); two appearances as the character Susan Klein, a reporter, on NBC's The Fresh Prince of Bel-Air, her TV sitcom debut; fellow comic Bob Goldthwait's movie Shakes the Clown (1992); and an episode of ABC's divorce-attorney series Civil Wars, Griffin's dramatic-series debut.

After starring in an HBO Half Hour Comedy Special, Griffin's first consistent public exposure came in 1996, when she was cast as the acerbic colleague of Brooke Shields' title character on the NBC sitcom Suddenly Susan. In 1998, Griffin starred in her first one-hour special, HBO's Kathy Griffin: A Hot Cup of Talk. She honed a comedy and television career that poked fun at her relatively modest place in the Hollywood hierarchy in a self-deprecating manner. She frequently appears in such self-consciously tacky projects as the reality show competition Celebrity Mole Hawaii — in which she won the 2003 edition after undergoing such experiences as walking over hot lava with her bare feet. She identifies her victory as the moment she became a "D-list" celebrity.

Griffin also has a secondary career in voiceover work, and has been featured on a variety of projects such as the Dilbert animated series and one of the Spider-Man animated series.