Lake County Circuit Court Judge Daniel Shanes found Baker, 24, guilty but mentally ill for the murder of his girlfriend's mother, Marina Aksman.
In his ruling, the judge said the issue was not whether the Deerfield man committed the crime, but if he was insane when he did it.
Dressed in a dark blue jumpsuit, Baker sat motionless as the judge read his verdict: The suburban man was found guilty of first-degree murder for beating Aksman to death with an aluminum baseball bat.
Investigators say Baker killed Aksman April 1, 2010, in a fit of rage after she forbid to see or date her daughter.
While prosecutors say Baker's attack was premeditated and willful, the defense argued that Baker, who has a history of mental illness, was in a psychotic state at the time of the attack.
"We disagree with the verdict, but it a very well thought of verdict," said defense attorney Ed Genson. "It was a very tragic case, a loving woman was killed and a young man who is extremely mentally disturbed."
"We're very pleased with the verdict. The victim's family, most importantly, is pleased with it as well," said Lake County prosecutor Ari Fisz.
Family members of both the victim and the defendant left court without comment. They said they might comment during the time of sentencing.
Now that he has been found guilty, Baker faces anywhere from 20 years to natural life in prison.
Prosecutors say they will ask for the maximum sentence.
The defense says it will appeal.
Sentencing could come sometime in the next three weeks.