Instead, she will go to a state mental hospital and not seek to be released for three years, according to local TV station WISN.
A Wisconsin jury says one of two teens charged in the 2014 Slenderman stabbing of a classmate was mentally ill when the attack happened.
Forensic psychologist Gregory Van Rybroek, giving evidence at the trial in Wisconsin of one of the defendants, Anissa Weier, said the delusion was so strong about the fictional creature that she "couldn't stop herself".
The decision was made after almost a dozen hours of debate between members of the jury.
Prosecutors contend Weier knew what she was doing was wrong, but that she went along with the plot to preserve her friendship with Geyser.
Investigators said Geyser stabbed Leutner 19 times while Weier urged her on, in a sacrifice to online horror character Slender Man.
Both Weier and Geyser told detectives they felt they had to kill Leutner to become Slender Man's "proxies", or servants, and protect their families from the demon's wrath.More news: Trump adviser says military option not preferred for NKorea
Jurors saw clips of Weier's police interrogation, during which she appeared to move from belief in Slenderman to doubt to a realisation that he wasn't real.
Maura McMahon, Weier's attorney, said she trusted the case uncovered that kids might be taking care of mental issues lost on grown-ups who have turned out to be excessively occupied with their own particular lives, making it impossible to focus and assets proliferate to help them.
Ms McMahon claimed Weier and Geyser had become obsessed with Slender Man, developing a condition called shared delusional disorder.
"At the moment of this awful tragic event, that she was confused and convinced that something real was going on with this very unusual character called Slender Man", he said. Westendorf testified at Weier's mental capacity trial that she believes Weier suffered from a shared delusional disorder that left her unable to conform her conduct to the law when she and Morgan Geyser tried to kill their friend, Payton Leutner, in 2014. But she said that if anyone could recover, it would be Weier. She found guilty to attempted the second-degree intentional homicide.
The Slender Man is a fictional supernatural character.
A teenager who allowed a girl to be viciously stabbed 19 times in a bid to please a horror character has avoided a jail term. "It was a choice and she needs to be held criminally responsible", the Fox News reported.