Robert Nelson was born in St. Louis, Missouri. His formal education ended when he began a photo engraving apprenticeship. He started flying in 1938 with Waco F-2s and was licensed in 1939. In 1940 he joined the RCAF. Two years later, stuck as a staff pilot at an Operational Training Unit for observers, he had seen his classmates head out as bomber pilots, which he wished to avoid at all costs. His vision always had been to fly fighters.

He was Court-Martialled for performing unauthorised aerobatics in a Westland Lysander. He was given 28 days detention and reduced to the rank of LAC (Private First Class). During his incarceration he received notes of commendation about his flying ability and his positive attitude from two of his former COs, one of whom requested that Nelson be assigned to his fighter squadron.

In November 1942 Nelson transferred to the newly formed USAAF, where he became a First Lieutenant.

Fate turned against Nelson on 22 April 1944. He and two other pilots were on a standby for giving blood transfusions to a needy patient, but the doctor released them, deciding to seek ground personnel for transfusions in case a mission might be scheduled. One was, and Nelson was chosen to go, flying as "Tail-end Charley". During the mission, near Kassel, Germany, his engine quit completely. He successfully baled out, landing in a tree, and in an abortive attempt to evade capture he survived five days eating Buffalo Grass, which became his nickname for the remainder of his year as a POW.

He was discharged in 1946 as a Captain. He had flown Spitfires, Thunderbolts, and Mustangs. He had six victories to his credit, and was awarded the Distinguished Flying Cross with two Oak Leaf Clusters and the Air Medal with three oak Leaf Clusters.