It was the second trial for Michael George, whose 2008 conviction was overturned because of errors by prosecutors.
A Macomb County Circuit Court jury found the 51-year-old guilty of first-degree murder, insurance fraud and possessing a firearm in a felony.
Prosecutors said George was cheating on his wife, Barbara, and having an affair with an employee.
They said he wanted to be rid of his wife and collect on about $130,000 in life insurance when he fatally shot her in 1990 in their Clinton Township shop, Comics World.
They said her husband tried to make the death look like it happened in a robbery.
Comic books valued at $30,000 were taken.
"Now that this is over, our family can go on with our lives, and Barb can rest in peace," Joe Kowynia, Barbara George's youngest brother, said after the verdict, according to The Detroit News. "He can rot in jail."
Defense lawyer Carl Marlinga called the verdict "an awful tragedy."
"I was convinced he was innocent. I think the alibi witnesses were solid," Marlinga said. "I will never understand why the jury chose to ignore that."
George moved to Pennsylvania in 1992 with his current wife and seven children and opened a Comics World store in Windber, about 70 miles southeast of Pittsburgh.
In an opening statement, Marlinga told jurors that a man wearing a Greek fisherman's cap and a man with a fake beard and mustache were outside the shop about the time Barbara George was killed.
Marlinga also said that Michael George was at his mother's home at the time.
First-degree murder carries a mandatory sentence of life in prison without possibility of parole.