McCullough, a former Washington state police officer whose name was John Tessier at the time of the killing, was questioned after Maria disappeared in 1957, the Chicago Sun-Times reported. At the time, he had what appeared to be an alibi, but investigators said they were able to break it in 2009.
Maria vanished while playing outside her house in Sycamore. Her brother, Charles, testified Monday his sister's friend, Kathy Sigman, came to the door and told him Maria was gone when she returned to the street after a brief trip to her own house to get her mittens.
Maria's body was found five months later dumped in a rural area with stab wounds in her throat.
Sigman, who said Maria was with a man when she left, identified McCullough in court and was the prosecution's key witness.
"The weight has been lifted from my shoulders. Maria finally has the justice she deserves," said the 62-year-old Chapman, adding she recognized McCullough as Maria's kidnapper as soon as she was shown his photo by state police in 2010.
"You never forget a face," Chapman said.
Police were tipped off when a half-sister of McCullough said her mother implicated McCullough in 1994 as she was dying from cancer.
"John did it, John did it -- and you have to tell someone," Janet Tessier said her mother told her.
"Never once did we think it was a neighbor," Charles Ridulph said after the verdict. "I think we could just not bring it in our hearts to think a neighbor could have created this horrible crime."
Judge James Hallock set a sentencing date of Nov. 30.