The other inmate, who was not identified, is serving 23 to 40 years for second-degree murder and a weapons offense, state spokesman Russ Marlan told The Detroit News Thursday.
Kilpatrick was transferred Wednesday from federal custody to the G. Robert Cotton Correctional Facility in Jackson, Mich., so he could prepare for a parole hearing in May. His first possible release date is July 24.
The ousted mayor rejected an offer of solitary confinement, Marlan said.
"Solitary confinement is what you do when you want to torture someone. No thank you," said Kilpatrick's lawyer, James Thomas.
Both Kilpatrick and his cellmate are classified as medium-security inmates.
The killer's classification was lowered after he served seven years, Marlan said. Cellmates are matched by security risk, not convictions.
Kilpatrick was transferred to federal custody last July when he was charged with 9 federal counts of fraud and income tax violations.
He is in state prison for violating his probation after a text messaging and perjury scandal that cost him his office.