After the first World War Haarmann returned to his native city of Hanover, having been discharged from the army. He suffered from epileptic fits, and these may have accounted for his degenerate social behaviour. He served small prison sentences for petty theft and indecent behaviour with small children. Haarmann lived in Hanovers thieves quarter, where he existed by stealing food and informing for the police.
In 1918 Haarmann did what most serial killers did in that era, he saw an opportunity for gain. Haarmann pretended to be a policeman and went to the railway station; there he lured young refugees with promises of food and shelter. In September that year he was joined by another criminal, 24-year-old Hans Grans. Once they had lured their victims to Haarmann's house in Neuestrasse, they killed them and sold their clothes. They also made a good profit by selling the meat from their victims at the local market. Haarmann killed 27 people in the space of sixteen-months, on average of about two victims a week.
On 22 July 1924, Haarmann was accused of indecent behaviour. His house was searched, and articles belonging to the missing refugee boys were found. The skeletons of twenty three people were discovered on a foreshore of a river, to which Haarmann confessed killing. Haarmann also told the police of Grans, who was arrested.
Haarmann was tried in 1924 and found guilty to killing 27 people, although by his own confession he thought he had killed about forty people. Haarmann was executed by a beheading and Grans, who acted as his accomplice, was sentenced to twelve years imprisonment.