A native of Redondo Beach, California, Nielsen attracted interest from the Arizona Wildcats and USC Trojans college football teams.
Because he believed the Arizona environment would be better for his asthma, he spurned the local Trojans and accepted an athletic scholarship to play for the Wildcats.
[1] In his first two years, under head coach Tex Oliver, he joined fellow future NFL running back Tom Greenfield as part of a team known as the "Blue Brigade".
[5] He accounted for the Giants' first score in a 37–21 loss to the Chicago Bears on October 27, with a one-yard run.
One newspaper reported that Nielsen went on to join "in a reconnaissance mission", along with "driving armored cars ahead of the Allied tanks to scout out Nazi positions.