McManus started working for the singer in 1993, in the wake of the first allegation of sexual molestation. Jackson, who was plagued with sexual-abuse claims before and after his death in 2009, was never convicted of any criminal wrongdoing.
But McManus said in an interview airing Friday on "Inside Edition" she believes Jackson committed the crimes.
Asked, "Do you think Michael Jackson was a child molester?" McManus replied, "Yes, I do.
"He groomed the little boys," she said. "I think he was getting them drunk."
McManus said Jackson warned her to keep quiet about what she saw.
"He told me, 'What happens at Neverland, stays in Neverland,'" she recalled, adding Jackson threatened to have her killed if she ever spoke out.
"He said, 'If you ever go on a talk show, or a TV show, we can hire a hit man and have your neck slit. They'll never find your body. We can hire a sniper to take you out," she said.
McManus testified against the singer at his 2005 child-molestation trial, but he was acquitted of all charges.
Wade Robson, a 30-year-old dancer-choreographer befriended by Jackson when he was a boy, described Jackson as "a pedophile and a child sexual abuser" during a recent appearance on the "Today" show.
CNN said Robson denied in testimony at Jackson's 2005 child molestation trial that the singer had sexually abused him.
Robson said on "Today" he was "psychologically and emotionally completely unable and unwilling to understand that it was sexual abuse" at the time but is now ready to confront his past.