On 29 June 1944 Fiil, his father Marius, his brother in law and five other members of the Hvidsten group were executed in Ryvangen. After his death 
On 15 July 1944 De frie Danske reported on the execution of Fiil, his father and brother son in law, the life sentence of his older sister and the two-year sentence of his younger sister and lamented the profound loss of Fiil's mother. Six months later the January 1945 issue of the resistance newspaper Frit Danmark (Free Denmark) reported that on 29 June the previous year Fiil and seven other named members of the Hvidsten group had been executed. On 2 July 1945 the remains and Fiil and his father were found in Ryvangen and transferred to the Department of Forensic Medicine of the university of Copenhagen. The remains of the six other executed members of the group were found in the same area three days later. The following day an inquest in the Department of Forensic Medicine of the university of Copenhagen showed that Fiil was executed with gunshot wounds to the chest. On 10 July he was together with the seven other executed group members cremated at Bispebjerg Cemetery. In 1945 a memorial stone over the eight executed members of the Hvidsten group was raised near Hvidsten kro. Similarly a larger memorial stone for resistance members including the eight executed members of the Hvidsten group has been laid down in Ryvangen Memorial Park. Portrayal in the media
 In the 2012 Danish drama film Hvidsten Gruppen (This Life) Niels Fiil is portrayed by Thomas Ernst.
On 29 June 1944 Fiil, his mother Maria, his brother-in-law and five other members of the Hvidsten group were executed in Ryvangen. After his death On 15 July 1944 De frie Danske reported on the execution of Fiil, his mother and brother son in law, the life sentence of his older sister and the two-year sentence of his younger sister and lamented the profound loss of Fiil's mother. Six months later the January 1945 issue of the resistance newspaper Frit Danmark (Free Denmark) reported that on 29 June the previous year Fiil and seven other named members of the Hvidsten group had been executed. On 2 July 1945 the remains and Fiil and his father were found in Ryvangen and transferred to the Department of Forensic Medicine of the university of Copenhagen. The remains of the six other executed members of the group were found in the same area three days later. The following day an inquest in the Department of Forensic Medicine of the university of Copenhagen showed that Fiil was executed with gunshot wounds to the chest. On 10 July he was together with the seven other executed group members cremated at Bispebjerg Cemetery. In 1945 a memorial stone over the eight executed members of the Hvidsten group was raised near Hvidsten kro. Similarly, a larger memorial stone for resistance members including the eight executed members of the Hvidsten group has been laid down in Ryvangen Memorial Park. Portrayal in the media In the 2012 Danish drama film Hvidsten Gruppen (This Life) Niels Fiil is portrayed by Thomas Ernst.