In a court filing, Mangione's lawyers argued that US Justice Department officials should be precluded from seeking the death penalty because of actions that had violated his due process rights.
They said these included "staging a dehumanising, unconstitutional 'perp walk' where he was televised, videotaped and photographed clambering out of a helicopter in shackles on his way to his initial appearance".
"Because of the blatant, intentional and damaging nature of this torrent of prejudice from multiple public officials, mainly the United States Attorney General, from the inception of this case through the grand jury vote on April 17, 2025, the death penalty indictment against Mr Mangione must be dismissed," the motion stated.
Mangione has pleaded not guilty over the shooting death last December 4 of Thompson, who led UnitedHealth Group's insurance unit.
Thompson was gunned down outside a Midtown Manhattan hotel where company executives were attending an investment conference.
Public officials condemned the killing but many in the country said they shared the frustration of Mangione, 27, with steep US healthcare costs and the power of health insurers to refuse payment for some treatments.
Concern about escalating political violence in the United States has grown since Thompson's killing, especially after last week's assassination of conservative activist Charlie Kirk.
Prosecutors with the Manhattan US Attorney's office have until October 31 to argue for keeping the death penalty as an option if Mangione is convicted.
He faces charges of interstate stalking and murder.
The case is overseen by US District Judge Margaret Garnett.
In seeking the death penalty for Mangione, US Attorney General Pam Bondi on April 1 called Thompson's death "a premeditated, cold-blooded assassination that shocked America".
Lawyers for Mangione have called that announcement "unapologetically political," and said prosecutors breached normal protocols by not first conducting a lengthy investigation or giving defence lawyers a chance to push back.
Mangione's next federal court appearance is on December 5, and Garnett has said she may then set a trial date for 2026.
If Garnett allows the death penalty and a jury convicts Mangione of murder, the same jury would consider whether he should be executed.
Mangione also faces nine state-level criminal counts including murder.
On Tuesday, New York state judge Gregory Carro dismissed two terrorism charges against Mangione.
New York's death penalty was declared unconstitutional in 2004 but the ban applies in state cases not federal cases.
Mangione faces up to life in prison if convicted in his state case.
The next hearing there is scheduled for December 1, and no trial date has been set.
