"For the rest of my life, I will be haunted by the horror and anguish Dylan caused," she wrote. "I cannot look at a child in a grocery store or on the street without thinking about how my son's schoolmates spent the last moments of their lives. Dylan changed everything I believed about myself, about God, about family, and about love."
The killers' parents have repeatedly declined to talk about the massacre. They gave depositions in a lawsuit filed by families of the victims, but a judge in 2007 sealed them for 20 years after the lawsuit was settled out of court.
In her essay, Susan Klebold wrote that she didn't know her son was so disturbed.
"Dylan's participation in the massacre was impossible for me to accept until I began to connect it to his own death," she wrote in excerpts released by the magazine ahead of Tuesday's publication. "Once I saw his journals, it was clear to me that Dylan entered the school with the intention of dying there. And so in order to understand what he might have been thinking, I started to learn all I could about suicide."
In a statement with the essay, Oprah Winfrey wrote that Susan Klebold has turned down repeated interview requests but finally agreed to write an essay for O. A spokeswoman for the magazine said Klebold was not paid for the essay, and there were no plans for her to appear on Winfrey's television show.
A spokeswoman for the Klebold family said there would be no further statements.
In the essay, Klebold said her son left early for school on the day of the shootings.
"Early on April 20, I was getting dressed for work when I heard Dylan bound down the stairs and open the front door. Wondering why he was in such a hurry when he could have slept another 20 minutes, I poked my head out of the bedroom. `Dyl?' All he said was `Bye.' The front door slammed, and his car sped down the driveway. His voice had sounded sharp. I figured he was mad because he'd had to get up early to give someone a lift to class. I had no idea that I had just heard his voice for the last time."
She said she had "no inkling" how sick her son was.
"From the writings Dylan left behind, criminal psychologists have concluded that he was depressed and suicidal. When I first saw copied pages of these writings, they broke my heart. I'd had no inkling of the battle Dylan was waging in his mind."