Mack entered her plea to racketeering charges on Monday, shortly before federal jury selection was scheduled to start in Brooklyn federal court.
The trial is expected to detail sensational allegations that the group recruited sex slaves for its spiritual leader, Keith Raniere.
Prosecutors have accused Mack of allegedly helping Raniere recruit women to a secret sub-society within the group.
Authorities allege the women were branded by a surgical tool with a symbol that resembled Raniere's initials.
Mack has said in an interview that the group emphasised self-discipline and self-empowerment, and she likened the branding to getting a tattoo.
The defence says the women were never abused.
Raniere and three co-defendants, including heiress Clare Bronfman and Mack, previously pleaded not guilty to conspiracy and other charges in the sex-trafficking case.
A judge hasn't ruled yet on requests by the co-defendants to get trials separate from Raniere.
Opening statements are scheduled for 29 April.
Additional reporting by agencies