James Crumbley, 47, was found guilty in his trial on Thursday, carried out a month after his wife Jennifer was found guilty on manslaughter charges stemming from the shooting.
James Crumbley faced four counts of involuntary manslaughter, one for each of the victims at Oxford High School in the 2021 shootings. Jurors began deliberating on Wednesday.
Both James and Jennifer Crumbley will be sentenced on April 9. Manslaughter carries a penalty of up to 15 years in prison.
The Crumbley's son, Ethan, was 15 at the time of the shooting at the school involving a semi-automatic handgun.
He pleaded guilty in 2022 to four counts of first-degree murder and other charges and was sentenced to life in prison without parole in December last year.
The United States, a country with persistent gun violence, has experienced a series of school shootings over the years, often carried out by current or former students.
The Crumbleys are the first parents to be charged with manslaughter in a child's school shooting.
"This is a very egregious and rare, rare set of facts," Oakland County, Michigan, prosecutor Karen McDonald told the jury during closing arguments on Wednesday.
McDonald said James Crumbley repeatedly ignored warning signs that his son was deeply disturbed, did not get him the help he needed, and did not do enough to safely store the firearm in the family home.
"He did nothing over and over and over again," McDonald said.
McDonald also presented the jury with texts that Ethan Crumbley had sent to a friend and journal entries he had written in the months leading up to the shooting, in which he talked about wanting medical attention and hearing voices, but he was worried his parent would be "pissed."
On one occasion, according to a text message to a friend, McDonald said that Ethan had asked James Crumbley to take him to the doctor, but his dad "gave me some pills and told me to suck it up."
Defence attorney Mariell Lehman argued that James Crumbley could not have possibly foreseen that his son would carry out a mass shooting.
"James had no idea that his son was having a hard time," Lehman told jurors during her closing argument, saying no evidence had been presented that James knew the contents of his son's text messages or journal.
'HELP ME'
Gun safety experts have said they hope the Crumbley trials serve as a wake-up call for parents to better secure weapons in their homes. About 75% of school shooters obtained the guns used in attacks from their own homes, according to government research.
According to prosecutors, James Crumbley purchased the handgun used in the attack four days before the shootings on November 30 in 2021.
On the morning of the shootings, a teacher discovered drawings by Ethan that depicted a handgun, a bullet and a bleeding figure next to the words "Blood everywhere," "My life is useless," and "The thoughts won't stop - help me."
The Crumbleys, summoned to the school that morning, were told that Ethan needed counselling and they needed to take him home, according to prosecutors.
But the couple resisted taking their son home and did not search his backpack or ask him about the gun, prosecutors said.
Both of the Crumbleys challenged that account in their trials, saying that teachers in the meeting mutually agreed that Ethan could remain in school that day, and that at no point did they think he posed a danger to fellow students.
Ethan was returned to class and later walked out of a bathroom with the gun and began firing, according to prosecutors.
