Niels Birkedal Hansen was born on 17 October 1909 in Århus, Denmark. He was the only child of Johannes Frederik Birkedal Hansen, a wholesaler, and his wife, Anna Asta Eliesa. Despite receiving a decent early education, graduating high school in 1924, his early adult life was marked by instability and a string of failed ventures. He worked briefly as a sailor before drifting between various professions, none of which he succeeded in.

On his 25th birthday, he married Mette Kirstine Nielsine in Åby Parish. A year later, they welcomed a daughter. However, his life took a darker turn by 1939 when he was convicted of fraud, receiving a three-month suspended sentence. His personal and financial troubles seemed to weigh heavily, and in 1940, as World War II engulfed Europe, Hansen left Denmark for Germany in search of work.

By the summer of 1940, Hansen had relocated to Hamburg, Germany. There, he began working with the Gestapo, the Nazi regime's secret police. He remained in Hamburg until 1943, after which he returned to Copenhagen, Denmark, now under German occupation. Fluent in both German and Danish, he was employed by the local Gestapo unit as an interpreter.

It didn’t take long for Hansen to rise in rank. With chilling efficiency, he formed his own unit within the Gestapo known as The Birkedal Group (Birkedalgruppen). This elite and feared unit specialized in tracking down Danish resistance fighters. But their operations didn’t end at arrests, interrogation was brutal, and in many cases, fatal. Hansen himself was the architect of much of the violence, conducting systematic torture using physical beatings, psychological pressure, and cold-blooded executions.

The Birkedal Group became notorious across Denmark for its brutality. After the war, intelligence suggested the group was responsible for nearly 60% of all arrests of resistance members. By 1945, as Germany fell and Denmark was liberated, Hansen went underground, disappearing as the rest of his unit was captured, tried, and sentenced to death.

Hansen eluded capture for two more years until 1947, when he was finally apprehended in Germany, the last fugitive of the Birkedal Group to face justice. Returned to Denmark, he stood trial for his wartime actions.

During the trial, he attempted to plead insanity. Observed in Vestre Prison, his behavior grew erratic and disturbing. While medical evaluations did indicate signs of psychological instability, the court ruled that his mental condition was likely a result of his own actions and did not exempt him from responsibility. He was found legally sane and convicted of 28 counts of murder and 133 counts of aggravated assault.

In 1950, Hansen was sentenced to death. On 20 July 1950, he was executed by firing squad in Christianshavn, Copenhagen.