Allison Mack was ready to testify against Nxivm’s Keith Raniere

“Smallville” actress Allison Mack was all set to take the stand against her beloved “Grandmaster” Keith Raniere during the Nxivm leader’s sex trafficking trial — but was never called to testify, The Post has learned.

The 36-year-old pleaded guilty earlier this year to charges of racketeering and racketeering conspiracy, and then began speaking with prosecutors and was prepared to testify against the Svengali-like guru, according to multiple sources.

Though Mack didn’t enter into a formal cooperation agreement like co-defendant and fellow sex slave Lauren Salzman, she was considered a “cooperator,” sources said.

It wasn’t immediately clear why prosecutors opted not to call her as a witness, and lawyers for Mack did not return messages seeking comment.

Raniere, 58, was convicted on all counts last week following a harrowing seven-week trial, in which prosecutors painted him as a calculating predator who led a secret master-slave group in which women were branded with his initials, starved, and ordered to have sex with him.

Raniere, who faces life behind bars, was Mack’s “master,” witnesses testified, and also kept her own harem of slaves.

Mack is facing up to 40 years at sentencing, which is currently scheduled for Sept. 11. Her cooperation could merit a more lenient sentence.

Raniere is slotted for sentencing Sept. 25.