Kathy studied at Crawford High School, and started her film career in the early 1990s, with several petty roles. Kathy landed her first major breakthrough screen role as "Sister Mary Patrick" in the film Sister Act in 1992. She reprised this character again in the film Sister Act 2: Back in the Habit in 1993. She is most primarily known as the voice of "Peggy Hill" on the comedy-drama series King of the Hill from 1997 to 2000.Najimy gave her best performance with the role of Olive Massery in the famous television series Veronica's Closet from 1997 to 2000.
Najimy is an advocate of Women’s liberation and is also known as an activist for human rights and gay rights from the age of 18. She uses her celebrity status to raise money and donate them to charities by making an appearance on several game shows. She has appeared thrice on Celebrity Poker Showdown and emerged winner of the season six tournament and donated the prize money of $100,000 to V-Day, an organization that is working actively to stop violence against women and girls. She also emerged a winner at the Grand Champion on CBS' Gameshow Marathon in 2006 and donated the prize money won of $100,000 to a charity named Girls Best Friend, a charity that helps unshackle girls. She is a pure Vegetarian and for supporting PETA, she was awarded Compassionate Action Award in 2014.
Kathy resides in L A along with her husband, singer/actor Dan Finnerty and their daughter, Samia Najimy Finnerty.