Mangione, 27, has pleaded not guilty to murder, stalking and weapons charges in connection with the fatal shooting of UnitedHealthcare CEO Brian Thompson.
US public officials condemned the shocking assassination but Mangione became a folk hero of sorts to some who decry steep healthcare costs and insurance practices.
He arrived in court on Friday wearing prison garb and shackled at the feet before US District Judge Margaret Garnett in Manhattan, who called an evidentiary hearing over whether police who arrested Mangione in Pennsylvania legally searched his backpack.
Mangione's lawyers argue prosecutors with the US Attorney's Office for the Southern District of New York should be barred from using evidence found in Mangione's backpack - including a 9-millimetre pistol, silencer and journal entries - because police illegally searched it without a warrant.
Prosecutors have argued that police officers in Altoona, Pennsylvania did not need a warrant because they had lawfully arrested Mangione for providing fake identification and were legally permitted to search the bag for dangerous items before transporting it.
On Friday, defence lawyer Marc Agnifilo questioned the deputy chief of the Altoona, Pennsylvania, Police Department about its procedures for searching a suspect's property after arresting them.
The deputy chief, Nathan Snyder, testified that if officers found something illegal like drugs or a firearm, they would "probably go get a search warrant".
Under questioning by a prosecutor, however, Snyder said that if officers found a gun they would continue searching for other dangerous items.
Garnett concluded the hearing without ruling.
She has scheduled jury selection for September 8.
Mangione's lawyers are separately asking Garnett to throw out an indictment against Mangione over purported legal deficiencies, or block prosecutors from seeking the death penalty over alleged violations of Mangione's constitutional rights.
Mangione has pleaded not guilty to murder and other charges in a separate case brought by the office of Manhattan District Attorney Alvin Bragg.
The judge overseeing that case is also weighing a request by Mangione's lawyers to suppress evidence from the backpack.
A trial date has not been set.
