German-born Mack, 36, is facing charges including sex trafficking, identity fraud and money laundering for her alleged role in running a sex slave cult in Albany, New York.
Fighting back tears, Mack said she had joined the organization, led by self-described spiritual guide Keith Raniere, a decade ago to find goal in life.
Mack is best known for her role as a young Superman's close friend on the series "Smallville".
Clare Bronfman, an heiress of the Seagram's liquor empire, arrives at the Brooklyn Federal Courthouse, for her trial regarding sex trafficking and racketeering related to the Nxivm cult in the Brooklyn borough of NY, U.S., January 9, 2019.
Mack was believed to be the leader of DOS, described as a "secret sorority" that allegedly brainwashed members, put them on starvation diets and beat them if they don't recruit enough "slaves".
In a statement, Ms Mack admitted to recruiting women by telling them they were joining a female mentorship group.
"I believed Keith Raniere's intentions were to help people, and I was wrong", she told a judge in federal court in Brooklyn, New York City, as she pleaded guilty to racketeering. There was also a very graphic element that involved the branding of Raniere's initials and her own on the bodies of women.
The guilty plea means Mack will avoid going to trial with Raniere and two women in NXIVM's inner circle, Clare Bronfman and Kathy Russell. Until the Salzmans' guilty pleas, officials and associates had repeatedly denied all wrongdoing and disputed allegations that it is a cult.
Her plea change also comes after NXIVM co-founder Nancy Salzman and her daughter Lauren Salzman pleaded guilty to racketeering and conspiracy charges. Defense attorneys insisted that any relationship between the cult leader and the women was consensual. Online notes that if convicted, Allison could face 20 years of prison for each count she is charged with.
Raniere, 58, was arrested on sex trafficking charges in March 2018, and is being held without bail.