Kjær was born in Hvidsten Mark on 2 April 1903 as son of tenant farmer Jens Christian Nielsen and 39-year-old wife Johanne Petrea Christensen and baptized Niels Kjær Nielsen in Gassum on the 21st of the same month. On 15 May 1905 his name was changed to Niels Nielsen Kjær in accordance with a royal authorization. In addition to being a member of the Hvidsten group Kjær was also a radio mechanic or radio dealer. The group helped the British Special Operations Executive parachute weapons and supplies into Denmark for distribution to the resistance. In March 1944 the Gestapo made an "incredible number of arrests" including in the region of Randers where a number of members of the Hvidsten group were arrested. The following month De frie Danske reported that several arrestees from Hvidsten had been transferred from Randers to Vestre Fængsel. On 29 June 1944 Kjær and seven 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 several members of the Hvidsten group. Six months later the January 1945 issue of the resistance newspaper Frit Danmark reported that on 29 June the previous year Kjær and seven other named members of the Hvidsten group had been executed. On 5 July 1945 Kjær's remains and those of five others from the group were found in Ryvangen and transferred to the Department of Forensic Medicine of the university of Copenhagen. The remains of the two remaining executed members of the group, Marius Fiil and his son Niels had been found in the same area three days before. Alternatively, his remains were recovered on or before 3 July because on that day an inquest in the Department of Forensic Medicine of the university of Copenhagen showed that he 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.
