In his childhood, he lived with his parents in a staff housing of the correctional facility in Berlin-Plötzensee. He graduated from high school on September 13, 1898, and entered the faculty of law of the Frederick William University on October 18, 1901. In the years in between one can assume that he served in the military. On July 12, 1905, he left the university without a degree, shortly before the end of the semester on August 15. The reason was his career in the police - he had entered the police service in 1904 and passed the examination to criminal police officer on May 30, 1905. Two days later he started as detective assistant and he was promoted to criminal detective on August 1. When Gennat entered the criminal police, there was no separate homicide division. It was only on August 25, 1902 that an on-call homicide service was created. That had not even changed when the Berlin police was reorganized on June 1, 1925. Only on the basis of Gennat's efforts was a homicide squad created. This event made possible his promotion to lieutenant inspector. He had been passed over for promotion beforehand owing to his massive criticism of the conditions in the criminal department. After creation of the Zentrale Mordinspektion the division achieved enormous success under Gennat's leadership. In 1931, the homicide inspection was solving 108 of 114 crimes making for 94.7 percent. Gennat himself worked on solving 298 homicide cases. His department was organized as one standing inspection team with two backup teams. The active team had one senior and one junior homicide inspector accompanied by 4 to 10 criminal police officers, a stenotypist and a dog handler. The backup teams had a senior and junior homicide inspector plus 2 to 3 police officers and a stenotypist. The composition of the active team changed every four weeks to ensure that each officer gained enough experience. Gennat reorganized much of the structure of how to investigate homicide. He developed most of the scheme that is known today as profiling. His work is documented in articles for the public like the 1930 publication "Die Düsseldorfer Sexualverbrechen" about Peter Kürten, where Gennat coined the term "Serienmörder". During the Third Reich, he was able to continue his work despite keeping a distance from the Nazi Party. Based on his success, he was even promoted to department director in 1934 and vice director of the Berlin police in 1935. He married criminal inspector Elfriede Dinger shortly before his death on August 20, 1939.
