The admission came in federal court in Manhattan, where Mangione said he was prepared to admit to stalking the insurance company CEO before firing the fatal shots.
"On the morning of December 4th, I shot Mr. Thompson in Manhattan," Mangione said, according to several news agencies.
He admitted that he had sent an email to the insurance company saying he intended to kill Thompson, that he had followed the CEO and that he had used a 3D printer to make a gun.
When asked directly by Judge Margaret Garnett whether he realized that his confession meant he could be imprisoned for the rest of his life, the 28-year-old answered briefly, "yes."
Mangione is facing charges in both federal and state court for the murder, and his lawyers argue that the admission that the stalking led to the death should result in the state murder charges being dropped.
Mangione has accepted full responsibility for Brian Thompson's death, says defense attorney Karen Friedman, arguing that a "single tragic event" should not lead to two indictments. She points out, among other things, that New York state does not allow double indictments for the same crime.
Sentencing following the plea in federal court is expected in December, and prosecutors say they are seeking life in prison.
"You shouldn't be able to become a celebrity for murdering," says prosecutor Jamie McDonald, referring to the fact that both the murder and Mangione have received a lot of attention.
Mangione has been praised in some quarters in the United States, and on Friday supporters gathered to show their support.
