Jennifer Crumbley, 45, is the first US parent convicted of manslaughter over a mass shooting carried out by their child.
Prosecutors accused her of being negligent in allowing her son Ethan to have a gun, and ignoring warnings signs over his mental health.
The boy's father James has pleaded not guilty in a separate trial on the same charges.
Their son, now 17, is serving life in prison for killing four classmates at Oxford High School in Michigan on 30 November 2021. Seven others were injured in the shooting.
Addressing the jury the judge said the decision was probably "the hardest thing you've ever done".
Crumbley stood with her bead bowed as the verdict was read in Oakland County court on Tuesday.
She was charged with four counts of involuntary manslaughter, each carrying a maximum sentence of 15 years.
Some relatives of those killed in the shooting expressed relief over the verdict.
"The People spoke!" Buck Myre, the father of Tate Myre, a 16-year-old killed in the shooting, told the BBC in a statement.
"You can agree or disagree with the people, but this is how the system is supposed to work."
Ethan Crumbley killed Hana St Juliana, 14, Myre, 16, and Madisyn Baldwin and Justin Shilling, both 17.
Ms Crumbley and her husband bought the gun their son used just days before the shooting, the court had heard.
She claimed she had been rebuffed by her son while trying to address his mental health issues.
An independent investigation published last year alleged multiple failures from the school system, including allowing the killer to return to class after being flagged for his strange behaviour, without checking his backpack.