9:28 AM PT -- Mangione pleaded guilty to two federal stalking charges related to Thompson's shooting death ... one count of interstate stalking resulting in death and one count of stalking through use of interstate facilities resulting in death.

He admitted to stalking Thompson to a healthcare conference and shooting him with a gun he printed on a 3-D printer. As Luigi pled guilty, Thompson's widow Paulette was visibly emotional and other members of his family were seen fighting back tears.

Mangione said ... "I shot Mr. Thompson in Manhattan. I understood that my actions would place him in fear of death or bodily injury. I knew what I was doing was illegal."

After Friday's court hearing, Thompson's family issued a statement saying ... "Today's guilty plea marks an important step toward justice for Brian and for our family. While nothing will ease the pain of losing him, we are grateful that the federal justice system has held the person responsible for this heinous act accountable. Now we look to the court to ensure that sentencing reflects the severity of this crime."

Luigi Mangione will have Brian Thompson's family looking on as he faces a federal judge Friday ... with the slain UnitedHealthcare CEO's loved ones sitting front row.

Thompson's mother, widow and several other family members entered the Manhattan federal courtroom ahead of Mangione's highly anticipated hearing ... taking their seats together in the first row.

Mangione is expected to plead guilty to the two remaining federal stalking counts stemming from the former UnitedHealthcare CEO's 2024 killing.