Kaczynski is a graduate of Columbia University.
After the anonymous Unabomber demanded in 1995 that his manifesto, Industrial Society and Its Future, be published in a major newspaper as a condition for ceasing his mail-bomb campaign, the New York Times and the Washington Post both published the manifesto, hoping somebody would recognize the writing style of the author.
David's wife, Linda Patrik, first suspected Theodore and urged David to read the manifesto when it was published. David recognized Ted's writing style, and the experienced criminal defense lawyer the couple hired notified authorities. On April 3, 1996, police arrested Ted in his rural cabin in Lincoln, Montana. David had received assurance from the FBI that his identity as the informant would be kept secret, but his name was leaked to the media. In addition, he sought a guarantee from federal prosecutors that Ted would receive appropriate psychiatric evaluation and treatment. The Justice Department's subsequent active pursuit of the death penalty for Ted, and Attorney General Janet Reno's initial refusal to accept a plea bargain in exchange for a life sentence, was seen by Kaczynski and other members of his family as a betrayal. Such a plea bargain was eventually reached, and Ted was sentenced to serve life imprisonment with no possibility of parole. Kaczynski has stated in interviews that notifying federal authorities of his brother's possible involvement in the Unabomber case was a painful decision, but that he felt morally compelled to do it in order to save lives that might have been taken had the bombings continued.
David Kaczynski received a $1 million reward posted by the FBI for the Unabomber's capture. The reward was funded by a Congressional appropriation for the Department of Justice and was, at the time, one of the largest rewards issued in a domestic case. Kaczynski told the Associated Press that he planned to distribute the majority of the reward monies to the bombing victims and their families, adding that this "might help us resolve our grief over what happened."
Prior to turning his brother Ted in to authorities, Kaczynski worked as an assistant director of the Equinox shelter for runaway and homeless youth in Albany, New York, where he counseled and advocated for troubled, neglected and abused youth. His brother's confrontation with the death penalty later motivated Kaczynski to become an anti-death penalty activist. In 2001, Kaczynski was named executive director of New Yorkers Against the Death Penalty (as of 2008, New Yorkers for Alternatives to the Death Penalty). While the mission of NYADP originally focused only on ending the death penalty, under Kaczynski's guidance, in 2008, it broadened its mission to address the unmet needs of all those affected by violence, including victims and their families. After leaving the NYADP, Kaczynski served as executive director of Karma Triyana Dharmachakra, a Tibetan Buddhist monastery located in Woodstock, NY.
Kaczynski was portrayed by Robert Hays in the 1996 television movie Unabomber: The True Story.
Kaczynski was portrayed by Mark Duplass in the 2017 television series Manhunt: Unabomber.
Kaczynski is also a practicing Buddhist and is a vegetarian. In 2009 he published an essay about his relationship with his brother Ted, from childhood to adulthood, which appeared in a collection of essays.