Prosecutors and defence lawyers had since argued over Loughner's mental competence to stand trial. Experts said he suffered from schizophrenia and delusions, and he was forcibly medicated with psychotropic drugs for a year.
At a court hearing in Tucson yesterday Loughner appeared in prison khakis with his hands shackled and head shaved.
District Judge Larry Burns concluded there was "no question he understands what 's happening today."
The judge asked if he clearly understood the charges, and Loughner replied softly: "Yes, I understand." He then replied "I plead guilty" as each of the charges was read.
The judge told Loughner: "You walked up to (Miss Giffords,) drew your pistol and shot her in the head at close range. Your intention was to kill her."
Victims of the mass shooting and their families filled one third of the court room. Miss Giffords was not present. The gunman's mother, Amy Loughner, wept in a corner.
Psychologist Dr Christina Pietz, who treated him, told the court Loughner had initially been shocked that Miss Giffords survived, and said to to himself: "Jared is a failure."
Loughner later expressed remorse, telling Dr Pietz: "I hurt people. I hurt my mom and dad. I wished I’d never had those thoughts. I especially cried for the child."
The psychologist said Loughner no longer believed television was sending him messages, and that medication and therapy had "made him human again."
As part of the plea deal Loughner waived his right to appeal and will be officially sentenced to life without parole at a hearing on Nov 15.
Loughner was also ordered to pay $19 million in restitution to his victims and agreed to pay what he could.
