The Medical Examiner's Office scheduled an autopsy Thursday, the Chicago Sun-Times reported.
Quinn was found in his chambers Wednesday morning and pronounced dead at Northwestern Memorial Hospital. P. Scott Neville, presiding judge of the appellate court, said Quinn appeared to have died from natural causes.
A graduate of the University of Illinois Chicago, Quinn received his law degree from John Marshall School of Law in 1980. He joined the state's attorney's office in 1981 and spent 15 years as a prosecutor, finishing as head of the public integrity unit, before running for the appellate court in 1996.
During his career, Quinn was sometimes exposed to the seamier side of Illinois politics. In 2006, he testified for the prosecution at the trial of former Republican Gov. George Ryan, who was convicted of corruption.
Quinn told the jury he had witnessed a 1992 conversation between Ryan, then Illinois secretary of state, and Jack O'Malley, the Cook County state's attorney, after two Ryan aides were charged with selling driver's licenses to people who had been convicted of drunken driving.
Ryan, after listening to a plan Quinn had devised to prevent that kind of scheme, turned to O'Malley and, using an obscenity, said "they're my guys," Quinn testified.
State Supreme Court Justice Anne Burke told the Chicago Tribune she and Quinn became friends while they were colleagues on the appellate court. She said he spent most of his free time with his wife, Susan, and their three children.