Bell was born and raised in Las Vegas, Nevada, the daughter of Tanzy and Ron Bell. Her grandmother was Romanian. Bell began studying improv at the age of eight. She graduated from Bishop Gorman High School in 2002. After high school, she moved to Los Angeles, California and became a member of the Groundlings.
An alum of The Straitjacket Society in Los Angeles, CA, Jillian auditioned for Saturday Night Live, and though she did not join the cast, she became a Writer for the show in 2009 for the thirty-fifth season. The same year Bell appeared in an episode of Curb Your Enthusiasm as an assistant who wears revealing clothing at work.
In 2011, she began to play "Jillian Belk" on the Comedy Central series Workaholics. Film Director Paul Thomas Anderson noticed her in Workaholics and cast her in a small role in his 2012 film, The Master and yet again in another small role in his 2014 film, Inherent Vice. She had a recurring role as Dixie in the fourth and final season of Eastbound and Down. In 2014, she appeared in 22 Jump Street alongside Jonah Hill and Channing Tatum. In 2015, she co-starred in Goosebumps and The Night Before.
In January 2016, Comedy Central debuted a new series, Idiotsitter, in which Bell portrays the constantly misbehaving daughter of wealthy parents; her character, Gene Russell, is serving time under house arrest, and is being mentored by Billie Brown (Charlotte Newhouse), who is hired by Gene's parents. Idiotsitter was launched as a web series in early 2014, before being picked up in June 2014 by Comedy Central for network airing.