Educated at the University of Manchester and St. John's College, Cambridge, Cockcroft was Jacksonian professor of natural philosophy at the University of Cambridge from 1939 to 1946. In 1932 he and Walton designed the Cockcroft-Walton generator and used it to disintegrate lithium atoms by bombarding them with protons. This type of accelerator proved to be one of the most useful in the world's laboratories. They conducted further research on the splitting of other atoms and established the importance of accelerators as a tool for nuclear research. During World War II Cockcroft was director of the Atomic Energy Division, National Research Council of Canada. In 1946 he became director of the Atomic Energy Research Establishment, Ministry of Supply, at Harwell, Berkshire, and was a chairman in the Ministry of Defence from 1952 to 1954. Cockcroft was knighted in 1948 and was created Knight Commander of the Bath in 1953. In 1960 he became master of the newly founded Churchill College at Cambridge.