Born October 25, 1879, in Hanover, Germany, Haarmann was the sixth child of a real-life odd couple. His father, a surly railroad fireman, was dubbed "Sulky Olle" by acquaintances; his mother, seven years her husband's senior, was an invalid. In early childhood, Fritz became his mother's pet and grew up hating his father, preferring dolls to the sports normally enjoyed by boys. Packed off to a military school at age sixteen, Haarmann was soon released when he showed symptoms of epilepsy. Back in Hanover, he was accused of molesting small children and was sent to an asylum for observation, but he escaped after six months in custody.

Thereafter, Haarmann earned his way through petty crimes, molesting children for amusement, on the side. Turning over a new leaf in 1900, he became engaged to a local girl but abandoned her for the army when she became pregnant. Honorably discharged in 1903, he returned to Hanover and successfully avoided his father's efforts to have him certified insane. A series of arrests followed, for burglary, con games, and picking pockets, before Haarmann's father set him up as proprietor of a fish-and-chips shop. Fritz promptly stole the business blind, but he was less successful when he preyed on strangers. Convicted of a warehouse burglary in 1914, he was sentenced to five years in prison. Upon parole, in 1918, he joined a Hanover smuggling ring and prospered, simultaneously working for police as an informer. On occasion, he would introduce himself to strangers as "Detective Haarmann."

Wartime Hanover was jammed with homeless refugees, and Haarmann had his pick of boys, enticing them with offers of a place to spend the night. Among the first was Friedel Rothe, age 17, whose parents learned that he had met "Detective Haarmann" just before he disappeared. Police searched Haarmann's flat, but came up empty. Six years later, he confessed that Friedel's head, wrapped in newspaper, was Iying on the floor behind his stove while officers poked through his drawers and cupboards.

Late in 1918, Haarmann was sentenced to nine months in prison on charges of indecency with a minor. On release, he found new quarters for himself, falling into company with 24-year-old Hans Grans, a homosexual pimp and petty thief. They became lovers and business associates, Haarmann adding new lines of used clothing and black market meat to the stolen items he sold for a living.

Together, Grans and Haarmann launched a wholesale scheme of homicide for fun and profit. Homeless boys were lured from the railway station, subsequently raped and killed by Haarmann (who informed police that his technique involved the biting of a victim's throat). The corpses were dismembered , sold as beef or pork, incriminating portions dropped into the River Leine. Grans took his pick of the discarded clothing prior to selling off the rest; one victim was reportedly disposed of after Grans expressed a wish to own his trousers.

Hanover police were strangely blind to Haarmann's murderous activities. On one occasion, a suspicious customer delivered some of Haarmann's meat to the authorities for testing, and the "experts" wrote it off as pork. "Detective Haarmann" further called attention to himself by visiting the parents of a boy named Keimes, found strangled in a Hanover canal, and subsequently told police that Grans had done the murder. Since the pimp was then incarcerated on another charge, police dismissed the tale and never bothered checking Haarnnann's interest in the case.

On May 17, 1924, a human skull was found beside the Leine; another was unearthed May 29, two more on June 13, but Hanover authorities dismissed the matter as a "practical joke." Their attitude changed on July 24, when some children discovered a sack filled with human bones, including another skull, on the riverbank. Panic erupted, with newspapers reporting some 600 teenage boys missing in the past year alone. Dragging the Leine, police recovered more than 500 bones, accounting for an estimated twenty-seven victims.

By coincidence, Fritz Haarmann was arrested during this period and charged with another count of public indecency. A routine search of his flat revealed copious bloodstains, initially dismissed as a result of his unlicensed butcher's operation. Homicide detectives found their first hard evidence when parents of a missing boy identified a coat, now owned by the son of Haarmann's landlady.

In custody, the suspect suddenly decided to confess his crimes in gory detail. Asked the number of his victims, Fritz replied, "Thirty or forty, I don't remember exactly." Haarmann's trial opened on December 4 and lasted for two weeks, the defendant grandly puffing on cigars, complaining that there were too many women in the courtroom. Convicted of twenty-four murders and sentenced to die, Haarmann was decapitated on April 15, 1925. Grans, his accomplice, received a sentence of twelve years in prison.