Prosecutors say Prince Cunningham lured Tyesha Bell out of her apartment before she disappeared.
The two had a child together.
Investigators say Cunningham lied to police and tried to cover his tracks after her disappearance.
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Bell's remains were found in Montgomery in 2020, and forensic testing showed she had been shot in back of the head.
Cunningham was indicted and arrested on June 14, 2022.
He faces up to 85 years in prison.
Cunningham remains in custody at the Kane County jail.
"For more than 23 years, Tyesha Bells family has lived with unanswered questions, unimaginable grief, and the pain of not knowing what happened to their daughter, sister, and mother. Throughout this case, they carried themselves with remarkable strength and dignity. While no verdict can restore the life that was taken from Tyesha, we hope this decision brings them a measure of peace and the assurance that her life mattered, her voice was not forgotten, and the person responsible has finally been held accountable. This verdict sends a message that no amount of time, deception, or concealment can erase the truth or diminish our commitment to pursuing justice for victims and their families. My thanks to retired Aurora Sergeant Jim Coursey, Detective Jeff Koenings, Detective John Munn, retired Lieutenant Greg Spayth, and the other officers at the Aurora Police Department for never giving up in finding out what happened. My thanks also to Victim Advocate Sheila Gray and to the additional law enforcement partners who helped locate Tyeshas remains and arrest the defendant," Kane County State's Attorney Jamie Mosser said.