[2] Mack's first major television role was in an episode of the WB/CW series 7th Heaven, in which she gained attention playing a teenager who cut herself.

In October 2001, Mack began starring as Chloe Sullivan (an original character created for the show), one of Clark Kent's best friends in the WB/CW television series Smallville.

[8] In 2002, Mack made a couple of appearances along with her Smallville castmate Sam Jones III in R. L. Stine's miniseries The Nightmare Room.

On March 21, 2015, she tweeted that she would be appearing in American Odyssey as Julia, who befriends Suzanne, the daughter of Anna Friel's lead character Sgt.

Throughout its existence, advocates of NXIVM characterized it as a benign multi-level marketing company selling professional and personal development courses, while critics described it as a cult.

"[20][18] In 2015, Raniere created a secret subgroup within NXIVM called "Dominus Obsequious Sororium" (DOS), of which Mack was allegedly second-in-command.

As a pre-condition for joining DOS, women were required to provide "collateral", which included nude photographs, damaging information about family and friends, and rights to their assets.

[17] Recruits were also controlled in several other ways, including requirements to seek permission, physical isolation, forced participation in "readiness drills", sleep deprivation, extremely restrictive diets, and being subject to corporal punishment.

[27][28] Former NXIVM member Sarah Edmondson stated in a 2017 New York Times exposé and a 2018 A&E special on cults details about DOS that she had been branded in an initiation ceremony at Mack's house.

[32][33] The federal indictment accused Mack of "recruiting women to join what was purported to be a female mentorship group that was, in fact, created and led by Keith Raniere.

[38][17] On April 24, 2018, Mack was released from Metropolitan Detention Center, Brooklyn on a $5 million bond and held under house arrest under the custody of her parents in California.

[45] According to a filing by the United States Attorney for the Eastern District of New York, Mack sat down for proffer sessions to assist the government in the prosecution of Raniere starting in April 2019.

[52] Days before the sentencing, Mack released a statement in which she repudiated Raniere, saying that her involvement with NXIVM was "the biggest mistake and greatest regret of [her] life" and expressed remorse in regard to those affected.

In addition to the letter, her attorneys asked for no jail time in consideration for Mack's remorse and her cooperation with Raniere's prosecution.

The lawsuit details allegations of fraud and abuse and charges of being a pyramid scheme, exploitation of its recruits, conducting illegal human experiments, and making it "physically and psychologically difficult, and in some cases impossible, to leave the coercive community.

"[13][59] In 2019, Catherine Oxenberg produced the Lifetime television film Escaping the NXIVM Cult: A Mother's Fight to Save Her Daughter with actress Sara Fletcher as Mack.

[60] Mack had a long-term relationship with Canadian actor Chad Krowchuk during the 2000s[61] when she was residing in Vancouver, British Columbia, where Smallville was produced.