Weier and Morgan Geyer baited colleague Payton Leutner into the forested areas at a recreation center in Waukesha, a Milwaukee suburb in 2014.
McMahon said Weier will spend three years at a mental health facility before her family petitions for her release.
On Thursday, the defense and prosecutors rested in the trial of Anissa Weier.
"Anissa was under the command and control of a delusional disorder". He could sentence her more seriously than the supplication understanding calls for, including up to a 25-year responsibility, the same as the most extreme jail time she could have gotten.
She described the internet character as tall and faceless with numerous tentacles. Fountain cut Leutner 19 times while Weier asked her on, specialists said. She believed he would hurt her and her family if she didn't help commit murder.
A lawyer for Anissa Weier said that her mind was "broken" when the attack happened, the Associated Press reports.
"She will have an uphill road", McMahon said. At one point before the attack, prosecutors claim Weier unleashed Geyser by saying "go berserk", according to the Journal Sentinel.
"She knew it was wrong". "This was a real being to this child and she needed to protect those around her". The girls felt that they were Slender Man's "proxies", and had to stab their classmate on behalf of Slender Man so that the character wouldn't hurt their own families.
The girl, identified in court only by her initials, Kn, testified that Weier then told her: "Don't worry, it's not you".
Last month, Anissa Weier, 15, had pleaded guilty to attempted second-degree homicide due to mental illness or defect in the stabbing as part of a plea agreement.
Circuit Judge Michael Bohren rejected an initial verdict presented at 8 p.m. because the jurors who agreed Weier had a mental condition weren't the same 10 who believed the condition prevented her from knowing her actions were wrong and conforming to the law.
Psychologists testified at a previous court hearing that Weier suffered from persistent depression and a delusional disorder linked to schizotypy, a diminished ability to separate reality from fantasy.
The verdict doesn't need to be unanimous, just 10 out of 12 jurors must agree.
Geyser has pleaded not guilty to being a party to first-degree attempted homicide. A jury trial on her case, which began Monday, was set to determine whether Weier was mentally ill when she committed the crime.