Captain Nielsen was the pilot of a B-26B Invader bomber with 13th Bomber Squadron, 3rd Bomb Group. On 11 February 1951, while on a night intruder mission, contact with the aircraft was lost. He was listed as Missing in Action and was presumed dead on 31 December 1953. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Nielsen was awarded the Purple Heart with Oak Leaf Cluster, the Korean Service Medal, the United Nations Service Medal, the National Defense Service Medal, the Korean Presidential Unit Citation and the Republic of Korea War Service Medal.