Born "Mary Lucy Denise Henner" in Chicago, Illinois to a Greek mother and Polish father. Marilu's father and brother changed the family name from "Pudlowski" to "Henner" for business purposes. Henner was raised on the northwest side of Chicago, Illinois in the Logan Square neighborhood. Her mother, Loretta, was president of the National Association of Dance and Affiliated Arts and ran the Henner Dance School ("disguised as a three-car garage") for 20 years. Henner took her first dance class at age 2.

While a student at the University of Chicago in the Hyde Park neighborhood of Chicago, Henner originated the role of "Marty" in the pre-Broadway Kingston Mines' production of Grease in 1971. When the show was discovered and moved to Broadway, she was asked to reprise the role; however, she chose instead to play "Marty" in the national touring company alongside John Travolta, who played "Doody." Additional Broadway credits for Henner include Over Here!, with Travolta, revivals of Pal Joey, Chicago, Social Security, and The Tale of the Allergist's Wife. Her first film appearance was in the 1977 sleeper-hit, Between the Lines, co-starring then-unknowns Jeff Goldblum, Lindsay Crouse, John Heard, and Jill Eikenberry.

Henner came to national prominence with the role of "Elaine Nardo" in the television series Taxi. She was the leading lady in the 1984 film Johnny Dangerously, playing love interest to Michael Keaton. She also appeared in Noises Off (1992) and in Man on the Moon (1999), a film about her Taxi co-star Andy Kaufman. Henner played both herself and her Taxi character. She also was on Match Game '78. From 1990 through 1994, she appeared opposite Burt Reynolds in the TV show Evening Shade, which also starred Ossie Davis and Hal Holbrook.

In 1994, she hosted her own short-lived daytime talk show, Marilu.

Henner has written eight books on diet and health, the most prominent being Total Health Makeover, in which she explains the virtues of a non-dairy diet in conjunction with food combining and exercise. She leads monthly classes on her webswite www.marilu.com designed to help people integrate these steps into a healthier, more active lifestyle.

Henner starred in the Brooks & Dunn video "You Can't Take the Honky Tonk Out of the Girl," in late 2003.

In 2005, 2006, and 2007, Henner was the host of the television series, America's Ballroom Challenge. Henner said on an episode of The Ellen DeGeneres Show, in early 2008, that she has never actually danced ballroom and would like to go on a season of Dancing With the Stars. She later hosted FitTV and The Discovery Channel's Shape Up Your Life, which is based on her books.

In her autobiography, By All Means, Keep on Moving, Henner discussed her romances with actors Travolta and Taxi costars Judd Hirsch and Tony Danza. Her first two marriages, to actor Frederic Forrest and director Robert Lieberman, ended in divorce. She married Michael Brown, a former college classmate, on December 21, 2006 before 100 people in her Los Angeles home. It was the second marriage for Brown, who has three children (Cassia Brown, Carine Brown and Michael Brown). Henner has two children, Nicholas Morgan and Joseph Marlon, from her marriage to Lieberman.

Henner was a contestant on NBC's first The Celebrity Apprentice, in 2008. She was fired by Donald Trump in the 8th episode, but was brought back to help fellow contestant Trace Adkins in the final task of the show.

Her eighth book, Wear Your Life Well, arrived in stores on April 8, 2008.