At the age of ten Dehmelt enrolled in the Berlinisches Gymnasium zum Grauen Kloster, a Latin school in Berlin, where he was admitted on a scholarship. After graduating in 1940, he volunteered for service in the German army, which ordered him to attend the University of Breslau to study physics in 1943. After a year of study he returned to army service and was captured during the Battle of the Bulge.
After his release from an American prisoner of war camp in 1946, Dehmelt returned to his study of physics at the University of Göttingen, where he supported himself by repairing and bartering old, pre-war radio sets. He completed his master's thesis in 1948 and received his Ph.D. in 1950, both from the University of Göttingen. He was then invited to Duke University as a postdoctoral associate, emigrating in 1952.
Dehmelt became an assistant professor at the University of Washington in Seattle, Washington in 1955, an associate professor in 1958, and a full professor in 1961. He conducted his work on ion traps from the University of Washington, where he remained until his retirement in October 2002. He was married to Irmgard Lassow, now deceased, and the couple had a son Gerd. Later Dr. Dehmelt married Diana Dundore, a practising physician.