Raymond Foster, 45, of the Louisiana town of Bogalusa, was sentenced to life in prison for second-degree murder of Cynthia Lynch.
A former KKK member testified that Foster shot Ms Lynch, 43, after she told him: "I want out", AP reports.
Foster apologised to the victim's mother, and his own family, in court.
"I hope you can find it in your heart to forgive me," he said.
Ms Lynch was shot dead in November 2008 a day after initiation rites in rural Tammany Parish, about 50 miles (80km) north of New Orleans.
Photographs taken by a witness at the intitiation ceremony show the group in white Klan outfits.
In one photo, Foster uses a sword to "knight" Ms Lynch; in another, they are shown embracing.
Disabilities
The victim's mother, Victoria Lynch, said outside the court that she forgave the killer.
"I feel sorry for both him and the entire family. It's something that never should have happened to a child like Cynthia.
"I'm relieved because Cynthia has got her justice and maybe now she can rest in peace."
Mrs Lynch said her daughter was born with a number of disabilities and had no formal education or holding a job, Louisiana's WWLTV reported.
The trial was stopped shortly after the first witness took the stand so Foster could make his plea.
BBC News
