Before he was sentenced in a Sycamore courtroom, McCullough, now 73, denied he was responsible for the stabbing death of the young girl, who had been his neighbor in Sycamore 55 years ago when his name was John Tessier, the Chicago Sun-Times reported.
"I did not, I did not kill Maria Ridulph. I did not," he said, turning to face the gallery where two of Ridulph's siblings sat.
McCullough, who could become eligible for parole in 20 years, was found guilty in September of kidnapping and killing the second-grader.
"We know in our minds he did it," Patricia Quinn, Maria's older sister, said after the sentencing.
McCullough criticized Judge James Hallock for not letting 4,000 pages of evidence collected during the FBI's original investigation to be entered at his trial.
"What purpose is served if you put an innocent man in prison?" he asked.
The evidence was disallowed because the FBI investigators and witnesses were dead or unable to testify.
The verdict is expected to be appealed.
The former Washington state police officer's career in law enforcement ended in the 1980s after a misdemeanor conviction related to sexually abusing a teenage runaway, the Los Angeles Times reported. He had been working as a security guard for a Seattle retirement home before his arrest.
Police were tipped off when a half-sister of McCullough said her mother implicated McCullough in 1994 as she was dying of cancer.