Gacy was born and raised in Chicago, Illinois. Badly educated, he worked briefly in Las Vegas, Nevada before returning to Chicago. He attended business college and began a moderately successful career at a clothing company. In 1964 he married and moved to Waterloo, Iowa, where he managed a Kentucky Fried Chicken restaurant belonging to his wife's family. In May 1968 he was convicted of sodomy and sentenced to 10 years. His wife divorced him. He was paroled in 1971 and returned to Chicago where he worked for a construction contractor. In 1975 he left to establish his own business, PDM Contracting. He remarried in 1975 and moved to the Chicago district of Des Plaines. He became a prominent local businessman, a member of the Jaycees and a Democratic precinct captain. His new wife divorced him in mid-1976.
No suspicion fell on Gacy until late 1978 when he was investigated following the disappearance of a teenage boy. A search of his house revealed a number of incriminating items related to other disappearances. In December 1978 Gacy went to the police and confessed. He claimed he had first killed in January 1972. He confessed to 33 murders, indicating where the bodies were in 28 of the cases—buried under his house. Most of the victims were young male prostitutes. Bodies were uncovered from December 1978 to April 1979 when the last known victim was found in the Illinois River.
On February 6, 1980, Gacy's trial began in Chicago. During the trial Gacy's plea of not guilty by reason of insanity was rejected and he was found guilty on March 13 and sentenced to death. On May 10, 1994 he was executed by lethal injection. His execution was a minor media sensation, and large crowds of people gathered on the grounds of the penitentiary where it occurred. In a display of what has been called "shocking bad taste," vendors sold T-shirts and Gacy merchandise, and the people cheered at the moment when Gacy was pronounced dead.
Attention has been paid to his poor relationship with his alcoholic father and to the head trauma and subsequent blackouts of his teens to provide some psychological basis for his acts.