De Forest, who was born in Council Bluffs, Iowa, was interested in science from the age of 13. His father, a congregational minister, wanted him to study for the Church, but De Forest refused, going instead, in 1893, to the Sheffield Scientific School at Yale University. His PhD thesis, Hertzian Waves from the Ends of Parallel Wires (1899), was probably the first PhD thesis on radio in America, and drew on the work of Heinrich Hertz and Guglielmo Marconi. While working for the Western Electric Company in Chicago, he developed an electrolytic detector and an alternating-current transmitter.

In 1907 De Forest patented the Audion tube, a thermionic grid-triode vacuum tube that was a very sensitive receiver of electrical signals. This invention was crucial to the development of telecommunications equipment. In 1912 he had the idea of ‘cascading’ these to amplify high-frequency radio signals, making possible the powerful signals needed for long-distance telephones and for radio broadcasting. His invention formed the basis of radio, radar, telephones, and computers until the advent of solid-state electronics.

Throughout his career De Forest pushed for the acceptance of radio broadcasting. He was not a very good business manager, however, and had to sell many of his patents. Later he worked on a sound film system that was similar to the one eventually adopted. In the 1930s he designed Audion diathermy machines for medical use and during World War II he worked on military research at the Bell Telephone Laboratories.