James P. Eaton, 39, now faces up to 40 years in prison for his role in the 1997 slaying of 14-year-old Amber Creek, the Daily Herald reported.
Assistant Racine County District Attorney Robert Repischak said Eaton was responsible for Amber's death despite the lesser charge.
The plea deal was given a month after Eaton's defense team made a motion to present evidence pointing to another suspect in the young girl's death.
Eaton has pleaded no contest to the crime, meaning he does not admit to causing the 14-year-old's death.
"(Eaton) has not actually come out and said 'I did this' or 'I did that,'" Repischak said Tuesday.
With a no contest plea, prosecutors admit to having enough evidence to win a conviction if the case were to proceed to trial.
Eaton's sentencing is scheduled for Jan. 20.
According to the Daily Herald Eaton's defense team declined to comment on Tuesday.
Amber vanished in January 1997 after leaving a Chicago juvenile home where she was living. Her partially clothed body was found in a Racine County, Wisconsin, nature preserve about two weeks later, with a human bite mark on her neck and a plastic garbage bag over her head. Eaton was arrested a month later on charges of first-degree intentional homicide and hiding a corpse.