In July 2006, DioGuardi was a judge in an Idol-like TV show called The One: Making a Music Star. The show debuted on ABC with the second-lowest rating ever for a premiere on a major American network and was abruptly canceled after just two weeks.^[15]

She joined the hit Fox television show American Idol as a fourth judge for the show's eighth season in 2009.^[16] She returned to Idol for its ninth season.^[17] However, she decided to leave the show on September 3, 2010 and did not return for the tenth season of Idol.^[18] She has had Idol connections in the past: in 2000, DioGuardi and former Idol judge Paula Abdul co-wrote the UK number-one single "Spinning Around", performed by Kylie Minogue. She has written many songs sung by Idol winners and alumni such as Kelly Clarkson, Allison Iraheta, David Archuleta, Carrie Underwood, Diana DeGarmo, Katharine McPhee, Kris Allen, Danny Gokey, and Adam Lambert. She co-wrote the American Idol season 8 single, "No Boundaries", which was performed by both finalists during the competition.

Kara also co-wrote with Jason Reeves a song called "Terrified" for Katharine McPhee's album Unbroken^[19], which was covered by Didi Benami, an American Idol contestant from Season 9, during Hollywood week. DioGuardi also recorded a video of herself singing "Terrified" with Jason Reeves on YouTube, which currently has over 1,500,000 views. Other than that she appeared on Hannah Montana as herself, in episode "Judge Me Tender".

After quitting American Idol, DioGuardi will take the role of head judge on Going Platinum, a new singer-songwriter, reality competition airing on Bravo.