Prosecutors suggested that the Rev. Gerald Robinson had a strained relationship with the nun, a strict taskmaster, and that he reached a breaking point that day.
Sister Margaret Ann Pahl was stabbed 31 times through an altar cloth, with the punctures forming an upside down cross, and her killer then anointed her with a smudge of her blood on the forehead to humiliate her in death, prosecutors said.
Robinson, now 68, had worked closely with Sister Pahl at the Mercy Hospital chapel, where her body was discovered on April 5, 1980.
Robinson was a popular priest, reports CBS affiliate WTOL-TV in Toledo. He was especially well-liked in the city's Polish neighborhoods because he delivered some sermons in Polish and heard confessions in Polish.
He had been an early suspect, but he wasn't charged until two years ago. His attorneys argued that the nun's underwear and fingernails had traces of DNA that that wasn't from Robinson, and that there were no witnesses to place Robinson at the crime.
Robinson, who wore his priest's collar throughout the trial, had no visible reaction as the verdict was read.
Thomas Osowik immediately sentenced him to the mandatory term of 15 years to life in prison, and a courtroom deputy handcuffed the priest and lead him away.
"Let us hope that the conclusion of the trial will bring some measure of healing for all those affected by the case as well as for our local church," Leonard Blair, bishop of the Toledo Diocese, said in a statement. "The diocese has remained steadfast in the work of the Church and its ministries throughout this trial, and will continue to do so."
Robinson's status is that of a retired priest and he continues to be barred from any public ministry, Blair said.
The jury deliberated for six hours following nine days of testimony during which witnesses linked a sword-shaped letter opener found in Robinson's room with the nun's wounds and blood stains found on the altar cloth that covered her body.
The case relied heavily on forensic evidence because prosecutors presented no direct evidence that Robinson killed Sister Pahl, the caretaker of the hospital chapel.
Two forensic experts testified that a dime-sized medallion with the image of the U.S. Capitol on the letter opener appeared to be the source of a faint stain on the altar cloth.
In a videotaped interview with police just after he was arrested in April 2004, Robinson said he was stunned when he walked into the chapel and the hospital's other chaplain accused him of murder.
Jurors watched the tape during the trial and also saw how Robinson, left alone in a small room for a few minutes, fold his hands and began to whisper in a barely audible voice. He whispered the word "sister" and then prayed again with his head bowed, at one point saying, "Oh my Jesus."
Since his arrest, allegations swirled that police did not pursue the case thoroughly because the main suspect was a priest and that the killing was some kind of ritual slaying.
Investigators reopened the murder case in December 2003 after the prosecutor's office received a letter about a woman's claims that she was molested by priests for years as a child. Among the names she mentioned was Robinson. Police have been unable to substantiate her allegations of sexual abuse.
Robinson was a popular priest in this blue-collar city of about 300,000, where a quarter of the residents are Catholic.
He was transferred from the hospital a year after the stabbing and became pastor at three parishes in Toledo. At the time of his arrest, he ministered to the sick and dying in nursing homes and hospitals.
"Obviously, I'm relieved," the victim's niece, Marilyn Duvall, 54, of Nashville, Ind., said after she watched the verdict on television from home. "I was just wondering which way it was going to go. You just never know."