Lieutenant Nielsen was a crew member of a PBM-5S2 Mariner patrol bomber with Patrol Squadron 50. On 10 November 1953, the aircraft was enroute to patrol duty over the Yellow Sea when it crashed off Cheju Island, South Korea. His remains were not recovered. His name is inscribed on the Courts of the Missing at the Honolulu Memorial.

Nielsen was awarded the Air Medal, the Combat Action Ribbon, 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.