Jack McCullough, a 71-year-old former police officer with the Washington state cities of Lacey and Milton, could be transported immediately to Illinois, his attorney Cailin Daly told reporters.
During a brief hearing before King County Chief Criminal Judge Ronald Kessler, McCullough, shackled and wearing a red jail jumpsuit, signed a waiver of extradition.
McCullough was arrested on June 29 in Seattle and charged in Washington state court with being a fugitive from justice and held in lieu of $3 million bail.
On July 1, he was charged by prosecutors in DeKalb County, Illinois with the murder of 7-year-old Maria Ridulph, who was last seen playing with a friend near their homes in Sycamore, Illinois, on December 3, 1957.
Her decomposed body was found on April 26, 1958, in Jo Daviess County, Illinois.
McCullough, then a teenager known as John Tessier, was a suspect in the crime but not charged by prosecutors at the time.
He told police he was on a train from Rockford to Chicago on the day Ridulph disappeared.
But a woman who had a relationship with McCullough told authorities last year that she had seen the train ticket and it had not been used, according to court papers.
McCullough's stepdaughter, Janey O'Connor, 33, said he wants to return to Illinois because he feels confident he will be found not guilty.
"The sooner he gets to Illinois and gets his trial, the sooner he can come home," she said.
"He's a strong person," O'Connor said. "His main focus is my mother."
McCullough's wife, Susan, did not attend the hearing.
A runaway teenage girl who met McCullough in the early 1980s accused him of sexually assaulting her, according to a probable cause statement filed by the Seattle Police Department for his arrest.
He eventually was convicted of unlawful communication over his interaction with the girl, and was fired from his position with the Milton Police Department, the statement said.