District Attorney Larry Krasner's office filed a brief questioning Common Pleas Court Judge Genece Brinkley's "impartiality," citing her decision to check in on Mill at a homeless shelter where he was doing community service and later criticizing him for not doing more.
Krasner said she improperly referred to her own observations at his sentencing hearing.
"Judge Brinkley personally assumed the role of investigator," the brief says.
Mill, whose real name is Robert Rihmeek Williams, became a symbol for criminal justice reform activists after Brinkley sentenced him to 2 to 4 years in prison for minor violations of his probation conditions in a decade-old gun and drug possession case in November 2017.
He spent months in prison before a court ordered him released in April of last year.
Krasner said Mill, 32, should get a new trial before a different judge. He said the court "abused its discretion when it imposed" the sentence on Mill. He had asked for a new trial but his request had been shot down. His appeal efforts continue.
Brinkley has denied any accusations she was biased against Mill during proceedings.
A message was left Wednesday with attorney A. Charles Peruto Jr., who represents Brinkley.
Mill's well-publicized sentencing became a lightning rod for calls to change state probation and parole laws. In January, he joined fellow rapper Jay-Z and the owners of the Philadelphia 76ers, Brooklyn Nets and New England Patriots to announce the formation of the Reform Alliance.
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