Born in Poland, little is known about Hopfield's upbringing. He was admitted to Syracuse University around 1913, and completed his A.B. degree in 1917 and then worked as an instructor through 1919. At Syracuse, he had worked as an assistant with Raymond Thayer Birge, who moved to University of California, Berkeley around this time. Hopfield was then admitted to Berkeley with a fellowship, and was awarded his doctorate for spectroscopic work in 1923. He became an instructor and assistant professor of physics at Berkeley, and was awarded a Guggenheim Fellowship in 1928 to work with Friedrich Paschen for a year in Berlin.
By the time he left Berlin in 1929, the stock market crash and depression had ended most new faculty hiring.  Purdue University appointed him as a National Research Council Fellow for two years.  He next was employed in creating the physics exhibit for the 1933 "Century of Progress" World's Fair in Chicago. This was followed by a position with the Libbey-Owens-Ford glass company in Toledo, Ohio.  With the onset of the Second World War, physicists were again in great demand, and he moved to Washington, D.C., to participate in war-related research.  The end of his career was spent at the Johns Hopkins University Applied Physics Laboratory and at the Naval Research Laboratory in the Optics Division in Washington, D.C.. On January 8, 1953, Hopfield died after a brief illness.
Hopfield's son, J. J. Hopfield, was born in 1933 and also became a noted physicist.