Mack pleaded guilty in 2019 to various crimes, including extortion and forced labor, when she was a high-ranking member of NXIVM, the upstate New York group led by Keith Raniere, who is now serving a 120-year prison sentence.
Mack appeared before U.S. District Count Judge Nicholas Garaufis in Brooklyn to learn her fate.
Based on federal sentencing guidelines, Mack faced 14 to 17 1/2 years behind bars, though prosecutors said the actress deserved less time because of her cooperation with investigators.
"She cannot undo what has been done, and she will have to live with the regret for the rest of her life," Mack's lawyers said in a court filing last week. "But Ms. Mack still holds the potential to be valuable to society — as a family member, as a friend, as a helper to those in need and as a cautionary tale."
Also in last week's filing, Mack, 38, said she understands the physical and emotional pain she inflicted on cult victims.
"I have experienced overwhelming shame as I have worked to accept and understand all that went on and all that I chose," Mack wrote.
Devoting "my loyalty, my resources, and, ultimately, my life to" Raniere, she said, "was the biggest mistake and greatest regret of my life."
"I am sorry to those of you that I brought into NXIVM," Mack continued. "I am sorry I ever exposed you to the nefarious and emotionally abusive schemes of a twisted man. I am sorry that I encouraged you to use your resources to participate in something that was ultimately so ugly."
Former NXIVM members have told investigators how Raniere established the secret sorority within NXIVM in which women were kept on starvation diets, branded with his initials and ordered to have sex with him.
Mack and other group leaders used nude photos and other compromising material to pressure members into complying, prosecutors have said.
Mack is best known for her work in "Smallville," which ran on the WB and then the CW networks from 2001 to 2011, playing Chloe Sullivan, the close friend to young Clark Kent.
This story first appeared on NBCNews.com.