Judge James Hallock pronounced Jack McCullough guilty Friday in a crowded courtroom in Sycamore, 60 miles west of Chicago.
McCullough was around 17 when second-grader Maria Ridulph went missing in the small town where they both lived.
He later enlisted in the military, and ultimately settled in Seattle where he worked as a Washington state police officer.
The case went cold but was reopened a few years ago after his old girlfriend provided evidence that called McCullough's alibi into question.
He was arrested on July 1, 2011, at a retirement home in Washington state where he worked as a security guard.
(Copyright 2012 by The Associated Press. All Rights Reserved.)