Prosecutors on Friday said he deserved heavy punishment.
“There can be no celebrity in assassination,” Jamie McDonald, the US attorney for the Southern District of New York, told reporters.
But ardent supporters of Mangione flocked to the Manhattan courthouse to show their support amid heavy security as a helicopter hovered overhead.
Relatives of Thompson were reportedly in court for the hearing and a man shouted “America loves Luigi” at them as they left in a convoy of black SUVs.
Insurance industry frustration
Thompson’s murder unsurfaced deep public frustration with the lucrative US commercial health care system, with some social media users painting Mangione as a hero.
He was arrested in Altoona, Pennsylvania, on December 9 following a tip from staff at a McDonald’s restaurant, after a days-long manhunt.
He had traveled to New York by bus from Atlanta about 10 days before the crime, US prosecutors said.
After checking into a Manhattan hostel with false identification, he allegedly performed reconnaissance near the victim’s hotel and the conference venue where the shooting took place.
Following his arrest, officers strip-searched him and went through property that included a handgun, a silencer and a jar of Skippy peanut butter.
Mangione also admitted in court Friday that he used a 3-D printer to make a gun and equiped it with silencer and magazine.
A checklist also found on Mangione contained entries including “intel check in” and reminders to purchase a camera from the Best Buy retail chain.
Defendants in the United States can be tried at both the state and federal level for the same crime, although the charges tend to differ — as they do for Mangione.