On Thursday, Cook County Sheriff Tom Dart says Hutton was discovered alive in Montana.
It was Hutton's sister who contacted the Cook County Sheriff's Department after learning about its efforts to identify some of Gacy's victims. Thursday's announcement comes almost 35 years to the day Gacy was arrested.
It took more than 40 years, but Robert Hutton finally reunited with family.
"He was surprised that they were still looking for him. You mean they're still looking for me? And I said yes they are," said Det. Jason Moran, Cook County Sheriff's Dept.
Hutton was found by the Cook County Sheriff's Department, which is still trying to learn the names of seven unidentified victims of serial killer John Wayne Gacy.
Hutton's family reported him missing when he was 21in 1972, around the time Gacy's killing spree began.
"Back in those days, missing people-- I don't want to say it was dismissed. If it was a high-profile one, obviously there was a lot of attention there, but, by and large, people thought, okay, he walked away. He took off. He'll come back," said Dart.
The sheriff's department's Gacy investigation drew the attention of Hutton's sister in Nevada.
She suspected her brother may have been a Gacy victim, and in many ways, he fit the profile. He was a frequent hitchhiker who traveled the country as a contractor.
"Some of his victims worked for him doing construction, specifically concrete," said Moran.
But Hutton had not been killed by Gacy. He was living in rural Montana.
"He just got caught up in the 70s lifestyle, so to speak, and after years went by he became a little embarrassed that he hadn't been in contact with his family, and I think that made it easier for him to dismiss them," said Moran.
Hutton is now 62 and has reunited with his father. He's planning soon to see the sister who never stopped seeking answers.
"If people who have questions or concerns about missing persons, if every one of them would just get their DNA swabbed, put it in the national database, you'd be amazed how many missing persons cases would be solved," said Dart.
Since Dart reopened the Gacy investigation in 2011, one of the unidentified victims, William George Bundy, has been identified. Bundy, a 19-year-old North Sider and Senn High School dropout, was among at least 33 people Gacy killed in a spree that lasted from 1972 to 1978. Gacy was executed in 1994.
Bundy was among 29 victims found in 1978 on Gacy's property at 8213 W. Summerdale in unincorporated Norwood Park Township. Four other Gacy victims were found in the Des Plaines River. Gacy told the police he dumped a fifth victim in the river, but that body was never recovered.
Seven missing persons cases have been closed and two cold cases unrelated to Gacy have been solved as a result of the investigation.
There are still seven unidentified Gacy victims. Anyone who believes their male relative might be a Gacy victim should call 708-865-6244.
The Sun-Times Media Wire contributed to this report.