Nelson made his Olympic debut in the 10,000 m at the 1968 Summer Olympics in Mexico City. He was not used to the hot conditions or altitude and his performance suffered as a result. In spite of being ranked eighth in the world for that event, he finished in 29th place with a time of 31:40.2 minutes. Although, he had not performed well, he was pleased to have represented his country at the competition.
Nelson returned to his studies at St. Cloud State in 1969, but the year after he suffered a serious back injury due to heavy lifting and the surgery that followed impaired his running ability. He made his last international appearance in 1971 and retired thereafter, although he continued running for pleasure. Nelson has retired from his job as a health teacher, although he still substitute teaches. He still serves as a girls track and field coach at South View Middle School in Edina, Minnesota.