Allison Mack is a free woman after serving 2/3 of her prison sentence -- and she should count herself lucky ... seeing how her ex-sex cult leader has a very long time to go.

The former 'Smallville' star was cut loose from a federal prison in California Monday -- this according to online records. It seems she was let out quietly, and it's taken some by surprise, as she was only 2 years into a 3-year sentence.

The facility where Mack was serving her time -- which began in September 2021 -- is the same place where Lori Loughlin and Felicity Huffman went for their roles in the college admissions scandal. In other words, it's a low-risk prison for non-violent offenders.

You'll recall ... Mack was convicted on racketeering and conspiracy charges as a result of her ties to the infamous self-help group, Nxivm -- whose top dog, Keith Raniere, was accused of recruiting/abusing women ... not to mention running a criminal enterprise in his ranks.

He was ultimately convicted as well, and had the book thrown at him ... a 120-year prison sentence. During Mack's criminal proceeding, she ended up catching a major break in terms of punishment after federal prosecutors asked a judge to go light on her because of her substantial assistance in nabbing Raniere and others in the sex cult.

Mack -- who was said to have been a key head hunter for Raniere during his reign -- has expressed remorse about her involvement in the cult ... which forced members to brand themselves, have sex with the guy and pledge blind allegiance to him in his operation.