He moved to Denmark on the Sheldon Traveling Fellowship from Harvard, where he began work as a nuclear physicist; in 1971 he became a naturalized Danish citizen. Mottelson and colleague Aage Bohr helped prove the theories of James Rainwater regarding the structure of atomic nuclei. The trio shared the 1975 Nobel Prize in Physics for their work.
Professor Mottelson is a member of the Board of Sponsors of The Bulletin of the Atomic Scientists.