Najimy was born on February 6, 1957 in San Diego, California, the daughter of Lebanese American parents Samia (née Massery) and Fred Najimy, a postal worker. She was raised Roman Catholic and attended Crawford High School.
Najimy's film career began in the early 1990s, with a number of off-beat minor roles, but her first major role was in the 1992 comedy Sister Act, in the role of Sister Mary Patrick. She reprised this role in 1993 in Sister Act 2. She also starred in Hocus Pocus with Bette Midler and Sarah Jessica Parker as the Sanderson sisters. In 1997, she lent her voice to Warner Bros. Family Entertainment as Tilly the Hippo in the film Cats Don't Dance, alongside actors Scott Bakula, Jasmine Guy, John Rhys-Davies and Don Knotts, and has also played a role on the Nightmare Ned video game. In 1998, she made a cameo in Bride of Chucky as the maid who discovers Diane and Russ' bodies. In 1999, she played the Stepmother in "CinderElmo" a primetime special for Sesame Street. In 2001, she co-starred in the hit comedy film Rat Race alongside John Cleese, Rowan Atkinson, Cuba Gooding, Jr., Whoopi Goldberg, Jon Lovitz and Seth Green. She guest starred in That's So Raven. She has made four movies with Goldberg (Soapdish, Sister Act 1 and 2, and Rat Race).
From film, Najimy expanded into television roles, including a dramatic recurring role on Chicago Hope. Najimy was part of the cast of Veronica's Closet from 1997 to 2000. She co-created and starred in the hit Off-Broadway show and HBO specials "Kathy & Mo", with friend and actress Mo Gaffney. She appeared as Mae West in the Broadway hit Dirty Blonde. She also portrayed Dr. Mildred Finch on Numb3rs. Najimy also provided the voice of the drama teacher, Miss Thespius, in the 1998 Disney show Hercules: The Animated Series.
In 2003, Najimy provided the voice of Margalo in Stuart Little: The Animated Series, taking over from Melanie Griffith. She is the co-creator and director of the musical revue Back to Bacharach and David, which ran in New York City in 1992 and 1993, and which she directed again in Los Angeles in April 2009. She also appeared in Drop Dead Diva. In October 2009, Najimy played a police officer investigating an attack on Wisteria Lane on an episode of Desperate Housewives. In March 2010, she appeared in the Ugly Betty episode "Million Dollar Smile", portraying Betty's new orthodontist, Dr. Frankel. In July 2011, Najimy appeared in TNT's Franklin and Bash episode "Bangover", playing a judge. Premiering in August 2015, Najimy appeared in Disney's Descendants as The Evil Queen. In the movie, she makes references to her previous role as Mary Sanderson in Hocus Pocus.
Najimy married comedic actor/singer Dan Finnerty on August 8, 1995, and together they have one daughter, named Samia, born in 1996.