Lee de Forest was born in 1873 in Council Bluffs, Iowa, the son of Anna Margaret (née Robbins) and Henry Swift DeForest. He was a direct descendant of Jessé de Forest, the leader of a group of Walloon Huguenots who fled Europe in the 17th Century due to religious persecution.
De Forest's father was a Congregational Church minister who hoped his son would also become a pastor. In 1879 the elder de Forest became president of the American Missionary Association's Talladega College in Talladega, Alabama, a school "open to all of either sex, without regard to sect, race, or color", and which educated primarily African-Americans. Many of the local white citizens resented the school and its mission, and Lee spent most of his youth in Talladega isolated from the white community, with several close friends among the black children of the town.
De Forest prepared for college by attending Mount Hermon Boys' School in Mount Hermon, Massachusetts for two years, beginning in 1891. In 1893, he enrolled in a three-year course of studies at Yale University's Sheffield Scientific School in New Haven, Connecticut, on a $300 per year scholarship that had been established for relatives of David de Forest. Convinced that he was destined to become a famous—and rich—inventor, and perpetually short of funds, he sought to interest companies with a series of devices and puzzles he created, and expectantly submitted essays in prize competitions, all with little success.
After completing his undergraduate studies, in September 1896 de Forest began three years of postgraduate work. However, his electrical experiments had a tendency to blow fuses, causing building-wide blackouts. Even after being warned to be more careful, he managed to douse the lights during an important lecture by Professor Charles Hastings, who responded by having de Forest expelled from Sheffield.
With the outbreak of the Spanish–American War in 1898, de Forest enrolled in the Connecticut Volunteer Militia Battery as a bugler, but the war ended and he was mustered out without ever leaving the state. He then completed his studies at Yale's Sloane Physics Laboratory, earning a Doctorate in 1899 with a dissertation on the "Reflection of Hertzian Waves from the Ends of Parallel Wires", supervised by theoretical physicist Willard Gibbs.
De Forest was convinced there was a great future in radiotelegraphic communication (then known as "wireless telegraphy"), but Italian Guglielmo Marconi, who received his first patent in 1896, was already making impressive progress in both Europe and the United States. One drawback to Marconi's approach was his use of a coherer as a receiver, which, while providing for permanent records, was also slow (after each received Morse code dot or dash, it had to be tapped to restore operation), insensitive, and not very reliable. De Forest was determined to devise a better system, including a self-restoring detector that could receive transmissions by ear, thus making it capable of receiving weaker signals and also allowing faster Morse code sending speeds.
After making unsuccessful inquiries about employment with Nikola Tesla and Marconi, de Forest struck out on his own. His first job after leaving Yale was with the Western Electric Company's telephone lab in Chicago, Illinois. While there he developed his first receiver, which was based on findings by two German scientists, Drs. A. Neugschwender and Emil Aschkinass. Their original design consisted of a mirror in which a narrow, moistened slit had been cut through the silvered back. Attaching a battery and telephone receiver, they could hear sound changes in response to radio signal impulses. De Forest, along with Ed Smythe, a co-worker who provided financial and technical help, developed variations they called "responders".