He earned his medical degree from the University of Zurich in 1910, and for a short time practiced psychiatry in Schaffhausen and Bern. In 1912 he taught classes at the university clinic in Krakow. In 1914 he became an assistant to Julius Wagner-Jauregg in Vienna, and in 1915 joined the Vienna Psychoanalytic Society. He remained in Vienna until 1932 when he emigrated to the United States and worked in Philadelphia and New York City.
While a student in Zurich, Nunberg assisted Carl Gustav Jung at the Burghölzli Psychiatric Clinic with word association tests. For several years in Vienna he taught classes on neurology. In 1932 copies of his lectures were published, and in the preface of the publication, an impressed Sigmund Freud wrote that it:
"contains the most complete and conscientious presentation of a psycho-analytic theory of the neurotic processes which we at present possess".
In 1955, Nunberg's lectures were translated into a book titled "Principles of Psychoanalysis, Their Application to the Neuroses".  Nunberg was an early advocate of required "training analysis" sessions for psychoanalysts in training. While in New York he was a member of the New York Psychoanalytic Society, and was its president from 1950 until 1952.