Eggert was born in Glendale, California. Her mother, Gina Duncan, is a British-born talent agent, and her father, Rolf Eggert, was a German-born canning company executive. Eggert's quest for stardom started when her mother entered her in beauty contests. At age five, Eggert was Miss Universe in the petite division. An agent saw her and offered her a role in a Johnson's Baby Shampoo commercial.
In 1980, at age eight, Eggert landed a role in the movie Rich and Famous, opposite Jacqueline Bisset and Candice Bergen, playing Bergen's character's young daughter.
Also in 1980, Eggert provided the voice of Dennis's arch snobby rival Margaret in Dennis the Menace in Mayday for Mother, which was the first animated adaptation in the Dennis the Menace trilogy.
Eggert had a recurring role on Who's the Boss? from 1985–1986 as Marci Ferguson, who was a friend of Samantha Micelli (Alyssa Milano). In January 1987, she starred on the situation comedy Charles in Charge as Jamie Powell. In 1989, she hosted the Nickelodeon Kids' Choice Awards with Wil Wheaton. That same year, she was the first guest star to appear on The Super Mario Bros. Super Show!. She starred as Alexa Adams in the film adaptation of Danielle Steel's Secrets which released in 1992. She was featured as one of the many lifeguards on seasons 3 and 4 on the syndicated series Baywatch. She appeared on the series from 1992 to 1994 and was originally supposed to be on a Baywatch spin-off (she returned to Baywatch for the reunion movie Baywatch: Hawaiian Wedding (2003)). She appeared on Married... with Children in episode "Enemies" (opposite of the series Friends) as Shannon (1996), intended to be a back-door pilot for a spin-off.
Eggert appears on the cover of the debut album Lemonade and Brownies of rock band Sugar Ray.
In 2004, Eggert played the role of Ginger in the first edition of the TBS network's The Real Gilligan's Island reality show. She appeared on Scott Baio Is 45...and Single (featuring her Charles in Charge co-star) in 2007.
On September 23, 2009, VH1 announced Eggert's participation in the winter 2010 season of Celebrity Fit Club.
In 2010, Eggert appeared on the Comedy Central Roast of David Hasselhoff.
In 2013, Eggert was added to the cast of Splash, the celebrity diving show on ABC. In April, it was reported she was injured during the taping of an episode. In the days following, she denied sustaining any serious injury.
In 2014, Eggert began a new career as the owner-operator of an ice-cream truck, describing it as a "Family run business bringing a fun, unique, and innovative twist to the ice cream man."
Eggert was engaged to Canadian actor Corey Haim, with whom she starred in Blown Away (1992), The Double 0 Kid (1992) and Anything for Love (1993). Eggert is credited with helping to save Haim's life at least once by taking him to the hospital to detox during a 'narcotic rush', although she has said: "I spent a lot of nights in emergency rooms with him. I don't think that I saved his life, I just think that I was there for him."
Eggert was married to actor Justin Herwick from 2000 to 2002. They have a daughter together, Dilyn.
She gave birth to her second daughter, Keegan, in July 2011.
On April 21, 2015, she appeared on the television series Botched for a breast reduction.