McAndrew, 27, was then taken back to the Regional Psychiatric Forensic Unit at the Norristown State Hospital.
The judge ordered him to undergo psychiatric evaluation before sentencing. McAndrew could be sentenced to prison or, if he needs more treatment, stay at the state hospital.
Either way, "he'll never be a free man," said McAndrew's court-appointed attorney, Paul A. Bauer III.
"It was the verdict that I expected," Bauer said. "I think the insanity defense is an extremely difficult defense to prove in the commonwealth of Pennsylvania."
First Assistant District Attorney Kevin R. Steele said one of McAndrew's two half-sisters was in the courtroom Wednesday, and told him she was relieved by the verdict.
"They had great fear of what he could do if he were released," he said.
During testimony earlier in the day, Steele had tried to chip away at the defendant's insanity defense.
As he cross-examined a forensic psychiatrist testifying for the defense, Steele said that McAndrew may have claimed insanity so he could inherit more than $1 million from his mother's estate.
"He's got a really, really powerful motive to lie," Steele said to Rocio Nell, a psychiatrist and the chief executive officer of Montgomery County Emergency Service.
Nell said she had not considered that in her evaluation of McAndrew, whom she found to be "as psychotic as they come" shortly after the killings and to have met the legal definition of being insane.
Under state law, a person cannot reap benefits as a result of a slaying he committed. So McAndrew wouldn't have collected an inheritance if he had been found guilty. He might have gotten it if the verdict had been not guilty by reason of insanity.
What happens with a verdict of guilty but mentally ill is unclear, said lawyer Joseph J. Hylan, appointed by the court to be McAndrew's guardian on issues involving his mother's estate.
"There has been no appellate decision construing the concept of guilty but mentally ill," Hylan said after the verdict. That decision will be made by an Orphans' Court judge.
McAndrew declined to testify; Bauer said his client barely even spoke to him during the trial.
During testimony and argument Wednesday, McAndrew repeatedly put his head down on the table in front of him.