Throughout her adolescent and teen years, Sigler starred in many productions including "The Wizard of Oz," "The Goodbye Girl" and "Annie." It was a tape of her performance in "Annie" that landed Sigler a manager and it was through this manager that Sigler got her big break. Her manager suggested she try out for a part on the television pilot "The Sopranos" thinking the musical-sounding title might offer Sigler a chance to showcase her vocal skills. But it was Sigler's sassy portrayal of the eldest daughter in a suburban New Jersey modern day mafia family that won over the show's producer and director. Early on, the show's producers actually wanted to yank Sigler and replace her with a blond actress but it was decided it would be too expensive to re-shoot all the scenes. So Sigler had secured herself as a made character in one of the most popular and critically acclaimed dramas of the decade.

In 1999, after filming the "Sopranos" pilot, Sigler was still in high school and a rough break up with her boyfriend. This disappointment led to dangerously obsessive exercise habits coupled with a drastically restricted diet. Sigler's drastic weight loss and unhealthy behavior led her to eventually admit she was anorexic and seek treatment. Though the producers threatened to not let Sigler work, she recovered from her disease and became a spokesperson for eating disorders for two national organizations. In the next two seasons of "The Sopranos" Sigler's popularity grew and she appeared in sexy photo spreads in several men's magazines. Sigler was also contacted by a talent scout after finally getting to show-off a little of her musical talent in a "Sopranos" episode where Meadow sang along to the TLC song "No Scrubs." She got a record deal and recorded the album "Here to Heaven" with producers TK and Desmond Child. The album was released in October of 2001 and Sigler co-wrote several of the songs on the album. The album was not a commercial success, though her video was premiered on MTV's "Total Request Live."

Sigler continued to tape episodes of the fourth season of the "Sopranos" and also branched out into film with a starring role in "Extreme Dating," a romantic comedy about extreme situations which lead to romance. Also in 2002, Sigler was cast as the lead in Disney's Broadway production of "The Beauty and the Beast."

While Sigler put her college career on hold after attending NYU in the fall of 1999, she did not rule out returning so that one day, she could have a career outside the entertainment industry. She planned on majoring in psychology and was interested in becoming a child therapist. However, Sigler continued to answer the HBO call to duty, and returned after a 21 month long hiatus, to reprise the role of Meadow Soprano for "The Sopranos" sixth and reportedly, final, season.