Her father George Remini, owner of an asbestos company, originates from Sicily. Her mother Vicki Marshall, a high school principal, is Jewish and originates from Austria. Her parents were divorced in 1977. Remini has one full sister, Nicole Remini (born 1969), and four half-sisters: Shannon (born 1984), Stephanie (born 1978), Elizabeth (born 1976), and Christine (born 1970). When Leah was 14, she left the school against the advice of her mother as she wanted to pursue a different occupation. At the beginning of her working life, Leah had a variety of jobs, such as waitressing, selling car insurance, and as a telemarketer for a solar heating company. Leah got her first speaking line as an actress on an episode of Head of the Class. On July 19, 2003 she married actor Angelo Pagan. A documentary of the wedding was aired on the VH1 series Inside Out. On June 16, 2004, she gave birth to a daughter, Sofia Bella. Remini stands 5' 3" (1,60 m) tall, and her weight has naturally varied widely since becoming pregnant, a fact she was heard to lament on an episode of Fat Actress.

Remini's entrance into television came in the mid-'80s, when she appeared as Carla Tortelli's daughter Serafina on two episodes of Cheers. In 1989, she played the lead on Living Dolls, a short-lived spin-off from Who's the Boss? In 1991, she had a recurring role on Saved by the Bell and starred in the short-lived sitcom The Man in the Family. She gained some popularity with her role as Terri Reynolds on the 1997 NBC comedy series Fired Up. She had several guest appearances on shows such as Evening Shade, Diagnosis: Murder, NYPD Blue and Friends. She also was in the Friends episode "The One with the Birth" (season 1). Leah Remini recently appeared on MTV Cribs, revealing her mansion.

Remini is a practicioner of Scientology In December 2004, she distributed Scientology materials to her fans. In December 2005, she helped promote the gala opening of Scientology's controversial "Psychiatry: An Industry of Death" Museum, which puts forth a conspiracy theory connecting Adolf Hitler to the psychiatric profession. Previously, on CNN, responding to criticism of Scientology, she said:

If somebody is going to get turned off about something because of what they read or heard, then that person's not smart enough to even enter a church. If you're really against something, then know what you're against.

In July 2006, Remini was one of the first few privileged Hollywood stars to see "Baby Suri," the daughter of fellow Scientologists Tom Cruise & Katie Holmes.