John Wayne Gacy Junior was born into a middle class Irish Catholic family on March 17, 1942, the second child of Marion and John Gacy Sr. He enjoyed an unremarkable childhood, apart from a playground accident aged 11, when a head blow caused him to have intermittent blackouts until he was 16. He hero-worshipped his father, despite the fact that he was an abusive alcoholic with a violent temper, who was virulently homophobic, and who would often accuse the young Gacy of being sickly and effeminate.

Having dropped out of high school, he found work difficult to come by, and he enrolled in business school where his flare for sales was soon recognized, and he excelled in his first post-graduate job as a management trainee at a footwear company in Springfield, Illinois. He also became a tireless supporter of several organizations that served the community, often dressing as a clown to entertain sick children in hospitals.

In September 1964, Gacy met and married co-worker Marlynn Myers, and his new father-in-law persuaded the newlyweds to move to Iowa, where Gacy was offered a position within a chain of fast food that he owned. Here again he developed his profile in the community, by doing volunteer work within the community organization, the Jaycees.

Gacy decided to plead guilty to the sodomy charge, and he was sentenced to ten years in prison.

In due course Marlynn gave birth to two children, a boy and a girl, and Gacy appeared to be living the American dream, with ambitions to become Jaycee president. Cracks began to appear in the idyll, however, when rumors began to spread that Gacy was making sexual advances to young employees within his father-in-law's franchise restaurants. In May 1968 he was indicted for allegedly sodomizing a teenage employee, Mark Miller. Gacy maintained his innocence, claiming that the sex had been consensual, and that there were factions within his Jaycee group that were fabricating evidence to spoil his chances of becoming president.

However, four months later Gacy was charged with hiring a young man, Dwight Andersson, to beat up Mark Miller, but Miller was able to escape his attacker and had Andersson arrested, who confessed to having been hired by Gacy. Gacy was charged, and forced to undergo psychiatric evaluation. He was deemed fit for trial, but unlikely to benefit from any medical intervention. Gacy decided to plead guilty to the sodomy charge, and he was sentenced to ten years in prison. His wife divorced him, and he never saw her or their children again. His adored father also died while he was incarcerated.

While in prison Gacy was a model inmate and, amazingly, after only 18 months in the Iowa State Correctional Facility, he was released on parole on 18 June 1970, when he returned to Chicago, where he spent four months at home with his mother.

His mother was impressed with how well her son had adjusted to life after prison, and bought Gacy a ranch-style house just outside Chicago, in which she maintained a 50 percent interest.