Police say Lindy Jones, 43, was behind the wheel during a deadly traffic stop in Queens in 2024.
Jones is charged with possession of a weapon after a second gun was found in the car.
He will receive five years in prison and five years probation when sentenced on July 14.
His passenger, Guy Rivera, fired the shot that killed Diller after officers approached the car to investigate what appeared to be a gun in Rivera's pocket.
Rivera, 36, was sentenced in April to 115 years to life behind bars.
Queens District Attorney Melinda Katz vowed that her office will continue to work to get illegal weapons off the streets.
"Lindy Jones drove around the streets of Queens with a loaded, defaced firearm that threatened the lives of everyone he encountered. We have seen firsthand the devastation illegal guns cause as we continue to mourn the killing of Detective Jonathan Diller at the hands of Jones' codefendant. That is why my office will continue to methodically rid our streets of lethal weapons and prosecute those who choose to possess them. I thank my Homicide Bureau for securing a guilty plea from this defendant - sparing the family the trauma of reliving the details of that terrible day - and ensuring that he is sentenced to prison for his actions."
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